I was thinking about the diversity debate some more and I realised it's like this. Female actors complain about how the good parts dry up as they age, after all, why cast a 45-50 year old woman to play the love interest to a similar aged male lead when you can just cast a younger, hotter actress.
It's the same with black actors - the only reason to cast a black actor ahead of a white actor is if the part specifically requires it