Aamer Rahman broke it down pretty well. I don't agree with everything he says but most of it is pretty on point.
As a whole, mainstream America has never been self-reflective about how it consumes its media. It always takes a vanguard of artists and writers on the fringe calling out sexism, racism, and overall bigotry to get these things moving. Unsurprisingly, these forward thinking movements get piled on with a bunch of insults: Why are you taking it so seriously? Why are you seeing problems where they don't exist? Why are you trying to create controversy out of nothing?
Blatant blackface is now something that's considered unacceptable in polite society. It wasn't always that way, and the people who were against it were shit on by mainstream America.
Aamer also linked a great article by Harry Allen on how white emcees are treated by the media. The reality is that mainstream hip hop is marketed towards white people, not black people. Anyone who's gone to a big hip hop concert can tell you this. If you were standing on stage for a Dr. Dre and Snoop Dogg concert, you'd be greeted by a sea of white faces.
And that also means that when a market-friendly white emcee hits the stage, the industry thrusts a disproportionate amount of effort into making them into stars. Eminem can rap his ass off. He's definitely one of the greatest rappers of his generation. At this point, he ranks as one of the greatest of all time period. But why was he getting so much media attention over a guy like Nas? Not that Nas is begging for change on the street by any means but they were both in the press at the same time (Eminem making his mainstream debut and Nas making his comeback), both were controversial figures, both were in high profile feuds, and both were about equal in skill. Actually if you ask me, Nas is the better rapper. But why did America embrace Eminem with both arms and Nas with just one?