Because Asian men are neutered in the Western media and public consciousness, but Asian women are sexualised and considered exotic. People haven't really seen Asian men kissing white women on regular television, let alone porn.
Jet Li's Kiss of the Dragon is a great example of this (although otherwise dull) -- it doesn't matter that Jet Li who saves Bridget Fonda from a horrible fate. They can develop a close friendship, she can be extremely grateful to him, but he's completely asexual in the way he treats her.
I have no idea why Japanese film-makers don't hire women of other races though, since a lot of Japanese guys I've met on my travels are fascinated with white women.