A good chunk of that is going to be because the 90's was around the time when digital tools became accessible, good, industry wide things, and we haven't had a kind of big musical innovation since that point, as far as the technology itself goes. That transition probably happened more noticeably in the 2000's, but you could tell it was happening over the course of the 90's for sure. The music industry has also not changed that much, we're still very much living in that reagan kind of neoliberal huge music label era, but that's kind of been around forever, so I kind of doubt that's been a major change from the 60's up to now. You could maybe say that streaming and the internet has changed music, and it certainly has, because now there are no gatekeepers, everyone listens to everything, and lots of artists put out like, a 10 minute single that changes styles six times so it might be propagated better online, instead of like a 90 minute experimental album. But then, there are more room for both of them, because people are more easily able to find what they want, and the latter was never gonna be mainstream anyways.

If I had to point out a larger genre shift, there has definitely been a large mainstreaming of rap and this kind of "pop country" more recently. You had those in the 90's, kind of infamously, but hootie and the blowfish does not really sound like modern country through some cultural progression that I don't really understand because I'm not brushed up on it. NWA and Tupac do not sound the same as modern rap, which has been getting a lot more of a "soft" kind of vibe, which I'd probably attribute to the influence of like, kanye, and maybe some lo-fi stuff like nujabes, and maybe just a mainstreaming of the genre at large. The subject matter has shifted, the tone has shifted, and the music itself has changed. Those genres would not sound the same, relative to their 90's counterparts.

The biggest thing I can think of that probably makes 60's and 70's music sound out of place next to 90's music is probably how hair metal got killed by grunge, which I couldn't really attribute to any one reason in particular. There's a pretty clear line between your rock acts, which have been going forever, and your later metal acts, and that line still exists with grunge, but grunge marks a kind of tonal shift. You'd also have to ignore the whole of disco and club music, that motown shit from the 70's and 80's, which died out pretty hard, but most everyone does that anyways, so who cares. I don't know if I've heard many 70's or 80's stations that actually play disco, certainly, not in proportion to how popular it was, usually they just play like. Stevie wonder, from what I've heard, shit like that. Or MJ. The thing you could probably derive from disco, from the 70's and 80's into the 90's, would probably be like, drum and bass, and eurobeat, stuff like that, and then you'd get stuff like daft punk later on which has a pretty clear connection to disco generally.

I dunno, this is all to say, shit has substantially changed in almost every mainstream genre I can think of in the last like, 60 years, from the 60's. Some stuff has remained pretty similar, and some stuff has had an almost cyclical nature, but that's just kind of the nature of music, I think.

The best radio stations to me today, the ones that keep me compelled, are ones that mix freely from all eras (including this one) and genres. Try BBC Radio 6 Music, or WPRB (from Princeton University), Soho Radio.

From what I hear on these stations, where DJs are expected to fearlessly put on whatever they like, it seems, the music of today sounds just as good as from any other era, but for me it's always been about discovery, and sometimes that includes being a little uncomfortable, I like to teach my mind a new groove every once in a while and it has been known to resist. It happens to all of us, I'm afraid, more and more as we get older.

That said, I can comfortably be against some music industry tendencies, there is no "pop utopia" in the past.
Last decade it's been software tools like autotune; in the 90s the "volume wars" began and frankly, most USA rock sounds too similar, all trying to channel Led Zep and Black Sabbath through a punkish filter; in the 80s many bands were overproduced half to death, submerged in sonic synthetic fluff, all the new studio toys abused, layer upon layer upon layer.

In the proper hands, these technologies can help a piece of music shine brighter, but in the hands of producers following the bandwagon - and that's always been the majority of 'em - everything ends up sounding the same, like neon ads all around you.

I think if you take today's pop music and do a side by side comparison with 90s pop you'd be surprised by how different they are. Not to mention, there are many many electronic genres and subgenres around today that have arisen in recent years.

Autotune, more explicit woman pop artists, mumble rap, whatever the fuck "thrift shop" is, country rap, k-pop.

Also noticed much older genres being modernized a bit with the likes of Tyler Childers (a little bluegrass-like) and All Them Witches (psychedelic).

here's an album from 2020.

https://igorrr.bandcamp.com/album/spirituality-and-distortion

You think this will fit in next to nirvana?