Well, the reason I say "that's just how attraction works" is because society defines beauty based on a standard, and every society is different. For instance, look back to ancient Greeks, Egyptians, etc.; studying their art, each had a different idealization of a human being. That idealization changed throughout time (at some point, large, "fuller" women were the ideal; before that, pregnant women were considered most beautiful), and so here we are, today, the year 2003.
Taking this example, I'd be willing to bet that the average black man will have different opinions about white women than the average white man; put simply, I think that black guys will find black girls most attractive, white guys think white girls are the best-looking, etc. If nothing else, you know that white guys don't usually like big booty, but black guys do, right? At least, where I live a white girl with a big ass is pretty much left untouched by everyone but black guys and hispanics.... Anyway, what do black women have more of than anything else? Large, plentiful asses.
Though the "melting pot" aspect of the U.S. does play a role (I am assuming you live in the States), it's still fairly segmented. There is some interracial dating, but it is not nearly as prevelant as the existence of a tendency to date within one's race. Therefore, I would immediately conclude that it's simply natural that you attract women of your own ethnic background than any other. I do--I always attract hispanic girls, but never a white girl. For whatever reason, I also attract Asian girls--can't explain that one, really; but I do attract fewer Asian girls than I do latinas.
It may also have to do with social stereotypes. Theorizing, I don't attract white girls because they probably see me as someone who'll end up gardening, cooking, etc.; that's stereotypical, but that's what they're used to seeing, day in and day out--Mexicans working with plants and at McDonald's. I've heard many black guys complain that white girls seem scared of them, thinking they'll be mugged or raped or whatever; nothing could be further from the truth, but unfortunately this is what they think about when they see a black guy. Again, I know I'm not going to be a gardener and I know you're not a rapist, but it's this mentality that probably keeps them away from you and I. That's not to say you absolutely *can't* attract a white girl (of course I've seen interracial couples and I know a few), but don't be surprised that you're having a harder time at it.
Well, that's my two cents.