Racism in the US is a little complicated but fear of the 'other' is probably the best simple explanation. A hundred twenty years ago the Irish weren't white, Italians weren't white. White for all intents and purposes was white Anglo-Saxon Protestant. You're hard pressed to identify someone who's grandparents were Irish or Italian today. They're white enough. I can possibly see Latinos becoming white enough in a hundred years as they integrate and marry white Americans. Ted fucking Cruz is Cuban when it suits him politically. But blackness doesn't integrate the same way. If you're half black you still look very black. And the genetic dominance of African features is a random and unfortunate continuing identifier of otherness. At least that's my take.