To be fair, there were some years there where Madonna definitely stirred up controversy with her behavior. Like when she had the parking cone bra, or when she put out "Sex" and whatnot. But by that point, people were used to her transitioning her look and persona. It's the same reason why people now basically overlook the pseudo-Nazi period that Bowie had with his Thin White Duke character. It was an act, and he'd already demonstrated that he was putting on acts repeatedly. Plus, artistically, he was putting out good or at least interesting stuff.
I also think that we've seen a trend in recent years of big "events" being purposely turned into "controversies" to generate publicity. Janet Jackson's "wardrobe malfunction." Madonna and Britney Spears and Christina Aguilera kissing on stage. Madonna and M.I.A. and Nicki Minaj doing crap like flipping off the camera in a live superbowl performance. Britney Spears and the snake. And now, Miley Cyrus twerking and tonguing her way into headlines.
I think what's happened in this case is that the whole thing just seems so...manufactured PURELY to generate controversy that it all comes across as disingenuous. And that's really the big thing. When Madonna or Bowie or Gaga or any of those types does something wacky, the wackiness may be focused on, but it still seems to "fit" them and it seems genuine, even as it's also contrived as an artistic act. But when Miley Cyrus does it, it seems somehow more calculated and knowing as a bid for attention rather than artistic expression. Other artists may perform controversial acts, but they somehow have more integrity than Miley's thing.
I think there's also another aspect of this that has to do with the execution of the act itself. At the same time that it seems too calculated, it also seems incredibly....amateurish and poorly done, like she isn't entirely comfortable doing it, or is just "acting out" rather than it being part of her persona or whathaveyou. It's somehow more juvenile than what, say, other "controversial" figures have done. Maybe it's how crude it was, maybe it's something as simple as lolling her tongue around, but it just seems...lame. Really, really lame.