Because
a) mainly white men have built the system of white supremacy
b) black people have done a lot to maintain their dominated role (black men shoot/hate successful other black men way more than that they focus time on ridiculing or fighting white supremacy..)
c) not all white people actively partake in the oppression, most of them are born in a hyper privileged world and until they date a black man barely realize there is a system created that gives them privilege
Obviously - it would be a way better look if he dated a black woman, but it is his private life and he shouldn't be with someone so that OTHER people feel happier.
I personally loved his speech and this does not diminish that.