I think there are at least two reasons.
One is as you stated the lack of feminism. Western men expect that most Asian women are feminine, family-oriented and will take care of the house. That they respect their husbands and understand that the role of a wife is different from the role of the husband. This is the image western men have, and in my experience, it is in many cases true.
The second reason might be that Asian women from poorer countries are seeking a better life. A "normal" western man is seen as wealthy and desirable. So as a western man you might get to pick from the prettier Asian girls, while in the west you wouldn't be rich or "desirable" enough to pick a pretty one.
I'm not convinced that there is a big group of white men who actually think Asian women are more beautiful than white women. Although many probably enjoy the look of Asian women. On the other hand, more and more white women are becoming overweight which is unattractive for most men, so Asian women, being mostly thin, might become more attractive.
As I've said above, it might be true that the most beautiful women in the world are white, but it doesn't matter because there are millions of women of every other race who are more beautiful than the average white woman. Anyone who exclusively gets involved with women of one race isn't doing it because the women of that race are the "most beautiful." It's just a personal preference.