That could be a completely valid question. But my thought stems from how you see athletes from plenty of different (non-English speaking) countries that come over to play in a North American league and end up learning some english. You see it a lot in hockey.
I've never tried learning Chinese, Vietnamese, Korean, Japanese...is it that the linguistic skills are so vastly different from an East Asian country to an English speaking one that it's just that much more difficult to learn? Is it easier to go from Russian, Spanish, Italian, French to English? Or...is it a cultural/attitude thing where American's and East Asian people don't bother to learn another language cause they don't think they need to...
Basically, is it cause it's really hard or cause they just don't want to.