I think the race aspect is the not-uncommon outcome of the author (who herself is not a member of a racial minority in her environment) creating characters similar to herself and her friends as the leads. Rowling first created Harry, the trio, the Weasleys, Dumbledore and a few others. Ron is modeled on a close childhood friend, Hermione is a parody of her young self, Snape after a hated teacher etc. Then she filled in the other characters, and used them to maintain a balance of representation of races, ethnicities, sexes and whatever else she felt the need to balance. So obviously members of minorities can't be in any of the central pairings. (But why couldn't she have at least Neville or Luna be members of minorities?)