I think most of it just stems from, as Ashley said, that there aren't a lot of other races in Japan. I'm white. If I made up a person in my head, the default skin color would be white, simply because that's what I am. Anything else would take conscious thought. This does not make me racist, but simply that my mind perceives myself as the norm or the default.

In the US, it's probably because the target audience being perceived as 15-25-year-old white males. But in Japan, I think it's just the status quo.