The reason people don't want a black Superman is the same reason you (I'm assuming) wouldn't want them to make Wonder Woman a man, or Lex Luthor into just some crazy piss poor homeless person, because it's central to their characters to be what they are. It might be racist, but the fact is Superman is white and will remain so because that's what he's always been, and it would require some pretty significant changes to his character if his race was changed, otherwise there would be no point in changing his race to begin with. Besides, Hollywood has never been good at casting minorities or women (or both) in a role without turning that role into a walking stereotype.