I rationalized that China got the better end of the Alliance and that the others we see most of the time in the series were those who got the short end. We don't see many Chinese fleeing the Alliance because they have the need.
A more sinister way of looking at it is how oppressive the government in China seems now and perhaps the series is actually symbolic of American idealistic values vs. that (not that we always live up to those ideals).
Mind you, I am not saying that is what Whedon necessarily had in mind but like I said my way to rationalize/interpret it. There are a number of possibilities of why there are so few Asians in the show and some are certainly more disparaging than others. It'd be interesting if any interviews ever broach this subject with the man who would know the answers.