Maybe it's because Asian guys are "prettier". They have more feminine features, IMO.
It may also have something to do with the fact that yaoi-sm is pretty big in Japan, and since we're all aware of the fact that there are tons of yaoi/shounen-ai titles around (and heck, we love them, even) we tend to be more receptive towards Japanese bishies instead of Caucasian ones.
Does this make sense? That's what I *think*, but you don't have to see it in this light. ^^