The reason why music seemed to change so rapidly in the latter half of the 20th Century, with whole new sounds, was because of technology. Music hasn't stopped evolving, but there's no longer anything that will sound new - for centuries that was the norm, but then we had electric guitars, synthesisers and so on given us whole new sounds. By the 90s it got to the point where computers could synthesise anything you wanted, and samplers could take any real world around you wanted to use. Where do you go from that?
Even if you tried really hard to come up with some new alien sound, chances are someone's already done it (even if it's some obscure noise or industrial band not in the mainstream).
There's also a paradox where sounds that we assumed would be the future become associated with the past. See how sci fi soundtracks have now largely moved away from electronic music, because it ends up sounding 70s. And whilst many films and TV assume the future nightclubs will be full of strange electronic music, it's nothing out of the ordinary for industrial or other alternative clubs since the 90s.