If MOBO simply does what it says on the tin and provides a platform for 'music of black origin' then why does it (largely) exclude mainstream rock and independent rock music?

As far as I can see, indie and rock can trace it's roots back to rock and roll and blues, which themselves find their genesis in Afro-American culture. In fact, I'm having a hard time thinking of many styles of music which aren't in some way, 'of Black origin'.