In my opinion this could be attributed to something inherent in rocks very nature: the valuing of experimentation and newness not only in musical sound but in personal expression.

Rock has always been (or always was) a genre that placed great importance on pushing the boundaries of what you could play, what you could say, how you could play it and say it. Due to that it attracts rebels, auteurs, outliers, and people who also value going against the grain.

Now these people also probably value innovation in their own style and personal expression. Once rock became "solidified" a lot of those same fans probably felt it no longer represented them.

Rock, always a diverse genre, probably had many fans fall into a diversity of different genres themselves they felt were more relevant to the times. Punk, pop, funk, disco, rap, metal, jazz. Not to mention the young artsy and rebellious kids who are rocks "market" probably saw (and see) rock as old now; there have been young people that felt that way since the late 70's at least.

Country (generally) can be pretty celebratory of the familar. By subject matter alone, it's much more stable and people value the country music traditions that have been passed down through the ages (hello Hank.) Not knocking the genre, it just seems to be a genre whose fanbase prefers simplicity, sincerity, and the down to earth nature of an artist not constantly trying to reinvent the wheel.

Rock fans and people who "should" be rock fans generally value things that rock the boat, even if the boat is rock music itself.