Because.. so called "black people" are individuals with individual minds.. intentions.. ideas.. experiences.. and like other people.. they swing and sway with the ideological winds. And it's from that context that they decide what they are about.. and what they support.. and oppose.. skin-color notwithstanding.
that said..
let me ask you this: "why do you think there is any difference between Clinton and Sanders?"
Obama got like 95% of the black vote in 08..
Skin color.