I think it has something to do with the history of oppression the African American community has had to endure and continues to endure nowadays; I'm talking about slavery, the civil right movement, police shootings, etc.
Of course, other minorities have to deal with racism, (that awful joke with the Asian kids was out of place

) and as someone mentioned, native americans have also been victims of a horrible history of violence and oppression. Perhaps with a latin, asian or native american host, the issue would have been handled differently.
It's a complicated problem and I don't want to get very deep into it, but I am absolutely sure African Americans are in no way mean spirited and they do not all share that "is not our responsibility to speak up for everyone else, especially since they wouldn't and don't speak up for us" attitude.