Mainly because in the west, asian dick is not really perceived as dirtier than caucasian dick so a scene where a pale pussy gets fucked doesn't automatically become kinkier/weirder if that dick happens to be attached to an asian face.You should wonder why there are very few AMWF in American and European porn.
Not so in Japan where caucasian dick is perceived as dirtier than japanese dick. Yet black dick is perceived as even dirtier than that so not that much need for caucasian dick from that angle. And then the other complexes towards foreigners (inferiority/superiority) don't really seem to come into play much in JAV (lack of foreign japanese-speaking acting talent?).