Asian men apparently have smaller genitalia than other ethnicities, so perhaps the anatomically-challenged Western male finds them to be a comfortable compromise. Usually fairly ugly, but not expecting much other than a stable, Confucian lifestyle.
Personally, I don't see the attraction.
I am currently reading a book about John Lennon, and of course he partnered-up with an Oriental woman. A crazy Japanese, and not that attractive physically, but there may have been other attributes that he liked - I am still working my way through said book.