I've wondered about this myself, but I think we may be making a false assumption here that the last 20th century is somehow the norm, and there are a lot of reasons to think that it's not...
1. The Collapse of Classical Music
Going into the 20th century, popular music consumption was dominated by what's now called 'classical' music. This probably only increased in the early decades of the century, as record companies made symphonies and operas, and the music of the great conductors and performers of the age - available to everybody. Film - particularly silent film - further enhanced the popularity of the tradition. Even outside the concert hall, popular music was largely of a piece with classical: Mahler may not immediately seem much like Music Hall, but the discontinuity is only there until you add in people like Gilbert & Sullivan (and a legion of similar but less memorable composers) who filled the gap. In the big picture, there was a pretty robust continuum between a popular song in the music hall, via lieder and round-the-piano domestic music and operetta, through into the lighter operas and the "pop concerts" into the really serious stuff. And indeed, many performers of popular songs - at least of the more serious kind - also performed opera arias and lieder. Even the American popular music scene was hardly differentiated - yes, there were distinct characteristics of 'jazz' music already, but they merged into ragtime and marching music and church music and musicals (ie operetta), and composers like Gershwin and Ellington saw no difficulty in bringing jazz inflections into their classical compositions.
But then the entire tradition collapsed. It had weakened by WWII, and imploded in the second half of the century - I think the last classical single to top the Billboard chart was in the early sixties, and the last album a few years after that.
You could say that that's due to the rise of pop music. But there's good reason to think that it was always going to happen. Postromanticism and modernism had reached a point where they were, I think in objective terms, extremely difficult for casual listeners. It was recognised within the tradition that there was a problem - hence the rise of neoromanticism and nationalism, and of irony and eclecticism in the work of Shostakovich and others. But the tradition just plunged further on, right into the morass of post-war neo-avante-gardism, from which there was no escape. And the thing is, this has all happened before, at least twice: when Ars Subtilior collapsed into madrigals around, say, 1420, and when the late Renaissance polyphony collapsed into the early Baroque monody in 1600. And arguably a third time when late Baroque ostentatious counterpoint gave way to early Classical homophonous, unornamented empfindsamkeit in the middle of the 18th century. Some sort of a shift toward more popular and simple music somewhere between 1900 and 1950 was therefore, while perhaps not inevitable, certainly predictable from the point of the view of the history of the tradition.
And as a result of development of unpopular classical music, a huge gap in the market opened up for new styles to fill. It's taken some time to fill them! Many people these days listen to more than one genre of pop music, effectively using the breadth of available genres to compensate for the emotional and structural narrowness of most of them - at first, the narrow confines of pop music could replace some functions of classical music, but were too homogenous to replace them all, and so more forms arose to fill more and more gaps. Now that people have more choices, there's less desire for a new thing to emerge to meet some unmet needs.
2. The aftershocks of the fall of Classical
Classical didn't disappear overnight. It staggered out, and that had a big effect on the history of popular music in turn. First you had attempts to reform within the tradition - people like Sibelius, for instance. Then you had attempts to move gently away from the tradition. Genres like jazz and the 'crooner' style (is there a proper word for that?) broke away from the academy, but still remained closely associated with it - popular singers still sang in a style similar to light opera, and they sang songs by classically-trained composers, often accompanied by classical orchestras and ensembles. The folk music revival, meanwhile, grew out of Nationalism and its desire to rediscover and return to ancient national traditions of music (and where necessary to invent them), and much of the work of collecting, popularising and recording folk songs was done by classical musicians and their fans.
A generation later, however, a lot of this was discarded as too close to the establishment, and the work of those musicians was recombined in a more genuinely chthonic way - not necessarily avoiding everything old (Elvis' cover of O Sole Mio was a massive hit, after all), but much more novel, and experimental, than what had come before. This being the sixties.
But since they'd broken away from the establishment and were experimenting on their own, they tried pushing their genre in new directions, which often involved resorting to classical music. This generation had still grown up with classical music, and many of its leading artists had grown up as classical music fans (like David Bowie) or had even trained as classical musicians (like Nina Simone, or half of the Velvet Underground), and it was possible for classical music to still hit the charts, either new classical music cunningly labelled as pop (like the Minimalist "Tubular Bells") or just sampled or covered or chopped up by pop artists (like Bach's "A Whiter Shade of Pale"). This knowledge of, and desire to emulate the sophistication of, classical music (cf the idea of the "rock opera" and "concept album" intended to prove rock's ability to stand alongside the operas and symphonies of the classical tradition) produced a distinct phase of pop music in the late sixties and seventies. That sophistication was in turn rejected in the late seventies and eighties in movements like punk and disco, by a generation who hadn't grown up in a culture dominated by classical music and who didn't see the appeal (or, in many cases, understand the techniques).
But aftershocks only last so long. Classical music has now become almost irrelevent [though I suspect it will continue to creep back into the mainstream in various ways], and pop music seems to have come, for now, to some sort of consensus about how much musical sophistication and continuity it wants. So that impetus for change has probably now passed, or at least become less urgent.
A huge part of how music sounds is down to instrumentation. Take a classical piece and play it with a mediaeval ensemble (shawms, crumhorns, tabors etc) and it'll sound mediaeval. Or mediaeval(ish), at least. Contariwise, when I was watching this year's Screenwipe, it took me a moment to realise that the classical piece they were playing in between the bits of Satie was actually a song by David Bowie, only reorchestrated.
Three massive developments in sound occured in the 20th century. From outside music, recording arrived. Recording didn't just mean you could get music out to the public more easily - it also changed the nature of music overwhelmingly. Take singers: before recording, singing technique was all about being able to project immense volumes to fill a concert hall (or even just a loud nightclub or pub), and that requirement meant that an opera singer and a popular music hall or big band singer were basically operating in the same sphere, in terms of techniques and in terms of sound qualities produced. Give a singer a microphone and they can do all sorts of different things. First they sang more quietly and intimately - crooning. Then they realised they could give up a lot of the old technique altogether, and you got the far looser, more personal quality we associate with pop music. Honestly, that's probably the single biggest difference in sound - get an opera singer to sing the Beatles, and it's not classical exactly but it's not pop music either. And its gone further. The Beatles began a wave of experimentation with screaming, shouting, growling, snarling and so forth. Others tried basically whispering. These days some singers just use talking.
And its not just the voice. Before recording, the guitar was a novelty instrument - heard occasionally in spanish-inflected chamber music or in domestic settings, a good instrument for a newcomer to try to play a song for himself at home on - but essentially useless in performance because of its inability to play loudly. Give that learner a microphone, however, and he can play right into your ear. Recording effectively brought the guitar into the centre of popular music, creating a whole new sound. And as a result, you could chuck out most of the rest of the band.
This had huge effects going forward for two reasons. First, by reducing the skill and sheer numbers required to make music, it opened it up to waves of amateurs. And, second, by obliterating the old assumptions about orchestration, it allowed a new era of experimentation, as people tried out different combinations of instruments that previously wouldn't have been viable. It seems that things have settled down a bit now.
This was all helped, of course, by the way that recording in turn enabled mixing.
Alongside recording, there was amplification. There needed to be: music ensembles designed for a recording studio struggled to be heard on tour in a concert hall. But because the guitar is a godawful instrument that human ears cannot bear to hear, this necessitated the invention of the electric guitar, which further developed the new sound. [basically, the sound from a guitar is awful enough that it's only tolerable because you can't hear it properly; when you amplify it, it becomes distinctly unpleasant. So people found ways to reduce the guitariness of guitars and to pick up a purer sound.]
Which links to the third innovation, which actually grew out of classical music: electronic music. During the 20th century various breakthroughs were made in electronic manipulation and generation of music, which were increasingly applied to pop music. But at first, all these different attempts had distinct and limited sounds, which helped change the timbre of music in different eras and styles in distinctive ways.
So a huge part of why the eighties are the eighties, musically, is the fact that synthesizers had reached a certain level of sophistication, and a certain accessible price, and like many new things they were seized on enthusiastically. And the nineties stopped being the eighties in large part because people decided they'd gone too far with the synths - and because synths became better and less obvious. And because synths were turned to new purposes - i.e. the basslines and rhythm, where the new synths could make sounds that were louder, lower but more attractive than any previous instrument other than the organ (which has always faced portability and accessibility issues).
Technology continues to improve. But, in a way, too far. Change and style are often driven by impossibility - the attempt to do something that can't quite be done. The sound of classic rock electric guitars comes out of an attempt to amplify guitars in a melodious way, which didn't quite succeed, but its failure was distinctive. The sound of eighties synths comes out of an attempt to make electronically generating music that sounds like a real instrument - and it failed, but the fact it failed (that those synths sound so weird and unnatural) is what gives that music its character. Now there is less and less reason to be weird (ie to change) - because you can get the sound you want now. Indeed, we don't even have to put up with new singing styles anymore, since computers can take any individual and easily homogenise them into an established, commercially viable style of voice... so any new changes in timbre will have to be intentional, brave, and visionary, and those things are in short supply.
4. The media revolution
The 20th century saw concert halls replaced as the main venue for music (and for music celebrity) by first radio and then TV. These mediums, in which a tiny number of people directly control access to the medium that is distributed to everybody, create a particular form of artistic culture. And that culture is inherently prone to change. The media oligarchs compete by finding the next big thing - and when it's "found", they use their oligopoly to insist that everyone else accept that this is the next big thing. We get a quest for the big innovator, followed by a slump of imitation as everyone copies what the innovator did (even the things that aren't really part of why they were a success). To break out from the homogenous mass of imitators, the oligarchs need to race to find the next innovator, to bring about the next era. Of course, it's not quite that clearcut. But I do think that this narrowness of the pipeline is likely to, by itself, produce a pattern of successive eras and generations in music.
But with the proliferation of radio and TV channels, and then with the internet, that oligopoly breaks down. There stops being the One Big Thing that everyone else has to follow. Artists can try to find a niche - rather than having to persuade the publishers and the networks that they were either a) a safe and uncontroversial bet or b) the game-beaking Next Big Thing. As a result, we're likely to see fewer major fads (that spawn eras, I mean - one-shot-wonder fads are probably going to be more common than ever) and a culture based less on periodic revolution and more on compromise and evolution. That won't stop change, of course, but it will probably slow it down.
It's like Venus. Venus has no continental drift, so it seems that its volcanic urges are filled by periodically replacing the whole of its crust. Revolutionary eruption, and then a new era of crust. Whereas Earth releases the pressure bit by bit, here and there, with mid-ocean ridges and subduction zones and whatnot, and there aren't necessarily distinct eras that areso easy to spot.
5. Social changes
The 20th century - I mean look at it! It was a time of many really radical changes in society. And now... isn't. Well, it might be, but so far it doesn't look to be as radical as the 20th, at least in the developed world. [I was talking to someone the other day, who was remembering someone they new. When this woman was a child, she remembered the bicycle first being introduced into her village; when she was an old woman, she saw men land on the moon.] For one thing, since the days of the early records an immense tide of lower- and lower-middle class people have entered the music-buying market, thanks to technological progress and rising incomes. That may be a shift that can't be replicated.
....well that's a bigger post than I intended!
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!