I'd assume it's multiple factors.

1. Cross-race effect

2. Romanticization of Western Societies

3. Bigger Penises (hypothetically speaking)

4. In the case of nearing the end of fertility cycle, desperate women are willing to marry a Westerner just so that women are shunned as much because single women over 30 are usually criticized sternly in Asian cultures, plus Westerners tend to earn more, and The Westerners they usually marry tend to know a lot about the culture.