I think a major reason is that rock is no longer subversive or edgy. The music that drives rebellion and controversy today is rap, and has been for quite some time. Think about the singular rock stars who not only drive controversy in the 90s, but were also pushed music forward. Axel Rose, Kurt Cobain, Billy Corgan, Eddie Vedder, Liam Gallagher, etc. By the 2000s we had nu metal which was wack but at least had rock stars. Once you get past 2005 or so where are the guys who were clearly That Guy, whether you fukked with the music or not? Compared to rap where suddenly Kanye, Wayne, Jay, Jeezy, TI, Drake, Kendrick, Travis, Uzi, etc etc were suddenly cooler or bigger or more controversial or more important etc than anything rock was producing.
Also the lack of a "great rock band." I couldn't even tell you who the biggest (active) rock band is today, not counting legends. I remember when Foo Fighters were that band for instance. They're still big now but like...who came after them and became that band? I have no idea.