Just to show a breh fiending for a white woman...that's the only reason.
Would have made wayyyy more sense to cast a big breh as Hagrid. Hell...even "he who shall not be named" would make more sense. Imagine a biracial Black/white kid that's the son of an American breh soldier.
He knocks up some white broad in WW1/WW2, leaves not knowing she's pregnant. Kid realizes he's got magical abilities but is fukked in the head because his dad doesn't know he exists and his mom abandons him in an orphanage.
Sirius Black as a breh would be funny for obvious reasons
. They chose the ONE mofo that will simp for a white woman the whole time AND THEN he becomes the magical kneegrow that sacrifices himself for a white boy.