Interesting thread.
I think the answer lies in both physical and social reasons.
Right now, we don't really know enough about the brain. This discussion isn't just a race thing, it's about aesthetics in general. Why are MOST people (of any race) just average usually? The best psychologist can give us now is "facial symmetry", and while I'm sure thats a factor, their has to be a reason why some people would have a fetish for one type of person, and absolute disgust for another.
With that said, I think it's worth noting that Africa is a very diverse continent, and not just in the split between North African Arabs and Black Africans, but within Black Africa as well. Black Africa is the most genetically and culturally diverse region on earth.
Most of the time, when you hear people talk about "black folks" their really just talking about African Americans/Jamaicans/Dominicans etc etc etc because diaspora blacks have gotten more spotlight.
^^^ Might not see a lot of Americans that look like this, but they are just as black as anyone else.