Without trying to answer for every white boy, or cum up with some overly pseudo-scientific model, I think there are a few main reasons, from my anecdotal experiences and observations.
First of all, on a purely aesthetic level we see the pinnacle of strength and the epitome of yielding joined together joyously and harmoniously. It is both beautiful and sublime. The skin color is important visually, but much like Yin and Yang, so much more is involved in these co-mingling concepts.
Second, but still a part of the first, we witness an obvious sexual selection dynamic play out, along the lines of Darwin or other evolutionary theories. I suspect that somewhere in our biological nature, there is an often forgotten backdoor conduit of pleasure wherein an inferior sexual specimen experiences lust while being left out of a sexual encounter dominated by his superior. Without going into the mechanics of how it could happen over the myriad generations, maybe there is a potential cuckold tendency, or identity, vaguely hardwired into some of us.
Third, there is still a lingering social dynamic at play, whether this be viewed positively or negatively. The taboo of it, whether succumbing to a hazardous temptation, or overcoming an obsolete barrier. I suspect this aspect is diminishing in importance quickly, however. And the first two aspects are all that is needed to further fuel the phenomenon.