I don't get it either . Maybe it's because the Asian girls look so young . Look at some of the pics that have been posted and my goodness those girls look very very young even though they are of legal .
We shouldn't be characterizing people by their racial stereotypes. However, the OP posed a serious question.
JH is right that in some of the pics that have been posted, the girls do indeed look very very young. (The CA definition of child porn includes pictures that show models who
appear to be under 18 displaying their genitals - even though the model is actually over 18.)
It's been said that the most common male fantasy is the schoolgirl fantasy. And it is unarguable that there are large numbers of chaps out there who appear to lust after underage girls. Even some terbites admit to being more inclined to the younger teenage look than to the adult woman look.
The explanation why Asian girls are so popular is indeed because so many of them, at 21, look (to westerners) as if they're 14. The explanation of their popularity is that the many many chaps who have a strong schoolgirl fantasy recognize that it is dangerous, under CA laws, to take the low road, so, in their large numbers, they take what they see as the high road - the Asian-girl road.