As some have mentioned it adds up to the ultimate in contrast and taboos combined.
I'm actually somewhat disappointed when white women get too tanned, granted its temporary and I know this but I just dig the paler the white woman. Not saying I don't like one's who tan but just don't get carried away in the tanning booth or sun.I'm dark enough for both of us.
Basically Its a base level attraction that makes me think of those white women as soft, silky, smooth and ultra feminine.