Thing is, I don't think rock is any less mainstream than it ever was. Even in their heyday, bands like Led Zeppelin, AC/DC, the Jimi Hendrix Experience and Nirvana were lucky to get the occasional Top 10 hit on the Billboard Hot 100. The singles charts have always been dominated by pop; hell, this was the biggest song of 1969:
I want another 90s rock revolution. So many classic albums, and it seems the rock scene made even the artists from other genres (mariah carey, whitney houston, garth brooks) step up their game. I want another music scene like that.
Maybe, It came to a point where EVERYONE and their Grandma just thought they could make a band and get popular and it became so repetitive, then it eventually just died out.
The same is currently happening with Pop music, so maybe Country will be the next big thing or something, who knows