First of all, we are not only talking about AOTY here - Kendrick has been nominated for 11 awards, 4 of which were rap related and another one a collaboration between him and one of the biggest popstars on the planet, maybe the biggest. And guess which he won? The 4 that are part of his genre and the one that had one of the biggest names of music attached to it, which is not even a real win to me since he was just a feature in her video/song.
He himself won in categories that don't go beyond his genre. It's like they are saying "Hey, you can win amongst your group of people but stay out of our game".
And I just don't see how you can count Kendrick, Weeknd, Bruno Mars, Alabama Shakes and D'Angelo in the same group of people just because of they share the fact they have different skin tones than white. It's not solely about skin color but about what these people do: Weeknd does pop-music that is inoffensive and can be played on shows and radios, he mostly features light-skinned women in his videos and fashion photoshoots; Bruno Mars does music that is as inoffensive as Abel's and has arrived in the mainstream ages ago; D'Angelo is the only one who goes in a similar direction as Kendrick but he soft-pedals and does R'n'B which is a genre I would argue that is not as dominated by lyrics as rap is. I am not familiar with Alabama Shakes but three YouTube searches later I think I have an idea of the music they are making.
And that's completely fine; everybody can do music as they want to, and I don't think anybody here is saying they don't deserve shit. But Kendrick? Kendrick raps about issues in society with a clarity that feels like a gut punch to a lot of people and delivered an album that sparked controversies, an album that is easily the blackest album of 2015: the music is black, the videos are anchored in black culture, the live performances have tons of Soul and Jazz elements in it. This is not about not feeling your face because you are a drug-addict, this is music about shit that is happening right now in America.
Kendrick is blacker than all of these artists you've mentioned and has the toughter mountain to climb because of it. If you honestly believe there is no difference in how people perceive both Kendrick Lamar or The Weeknd, then I'm not sure what to tell you.