It's not racism. It's old people. The old white people who vote on the Grammys don't know shyt about rap and only listen to specific genres. This applies to rock and metal too. They don't give a shyt about those genres.
So what happens is that name recognition trumps quality. If Stevie Wonder released a metal album he would win Best Metal Album no matter what, solely because old cac voters know who he is. Every few years the older voters get their way and give AOTY to an established artist's album - Herbie Hancock won a few years ago, Robert Plant won, etc. Regardless of whether it is considered AOTY quality.
I know somebody who is a Grammy voter. She's 60+, a university teacher, and is a pop music historian. Doesn't listen to rock, doesn't listen to metal, doesn't listen to hip hop, etc. Her grandson (my friend) picks those selections for her. Haven't spoken to him about this year's Grammys but last year he said he was 90% sure GKMC came in third place for rap AOTY. Behind Macklemore and Hov.