Considering that human brains evolved to distinguish between human faces, the fact that they notice more diversity between human faces than between chimpanzee faces doesn't really tell us much.
You'd have to do a much more exact survey of facial features than just looking at a few pictures and saying "Which ones seem more diverse?" before you can answer the question.
That doesn't mean I think that human faces aren't more diverse. I just don't know one way or the other.
If it is the case that human faces are more diverse, then I guess we might be faced with your question of why. There are all sorts of explanations that we could come up with, but it would take a lot more effort to actually test them, and I think that in the short term the results would likely be ambigous, though eventually we might figure it out.
So, to answer the OP, I don't know that there is a difference, and if there is, I don't know why. Aren't I helpful?