Plenty of reasons. To begin with, seeing how biased the Western media are when they report anything related to China is both appalling and disappointing. The impression of what average American has about Chinese government, Chinese people and their relation is light years away from reality.

Moreover, people immigrating to US from China tends to be more pro-democracy than average Chinese, but we got disillusioned after seeing how things worked out in American politics.

The life here is quite unsatisfactory for lots of Chinese, as there are many aspects of China making it a better place to live in compared to US. Can't speak for other people but frankly if I could have got a job with similar pay and similar workload in China, I wouldn't even hesitate for a second that I'd move back.