It really is just advanced racism, honestly.
The people who have that fetish view black people as below them, but get turned on by objectifying them as "Bulls."
It traces back to the 1800's, where slave owners would select certain male slaves as "bucks" and use them to breed "better" slaves.
People like that view BBC cuck porn in an almost similar way to bestiality stuff, where they get turned on by seeing a "pure" white girl getting "defiled" by a "beast".
Same reason why said fetish is most popular in the southern states of the U.S; the people who hate a group the most, somehow also fetishize them the most too.