My guess is that their economy can’t handle a no-limit policy. Willing to listen to other answers, though.
The Chinese recognize that a huge, growing population is hurting their ability to increase wealth and reduce poverty. In contrast, the US is importing poverty thru its immigration policy. Our population has increased by almost 30 million since 2000 or more than the combined populations of 19 states. 75% of the increase is due to immigration, legal and illegal.