China to hold nationwide survey on population changes as birthrate plummets (theguardian.com)

It's basically the same reason everywhere else - economics. People spend more time working and kids cost a lot of money. Perhaps it's even worse in China because the current married generation have to support 4 parents by themselves due to the old child policy. Imagine two adults supporting 4 elderly parents + kids. Having kids is luxury.

It turns out that instead of worrying overpopulation, the world should have been more worried about population collapse.