Radiohead briefly seemed like they could seize the mantle to me, but they rapidly moved in a direction too foreign to mainstream audiences to have staying power.
I think a lot of music with a hard political/social edge isn't subtle enough to hit the mainstream. Kurt certainly had a message, but I think the presentation of it through the music was ambiguous enough that it slipped through. I also think that in the last decade Americans have become less open to criticism than possibly any time in history. So cultural conservatism + lack of subtlety = nothing breaking through.
Also keyed into the cultural conservatism, I think that mainstream audiences haven't really been ready to listen to anything particularly challenging. Even when hip hop was holding sway, I would say many, if not most, of the biggest hits used hooks from past popular songs.
Also - the biggest reason I would say that Nirvana still holds popular imagination is because it's a moment frozen in time and a voice that's not around to comment anymore. If Kurt had lived and kept running his mouth, I'd say it's likely Nirvana would have fallen out of favor too.