Because two years of lockdown policies genuinely is psychologically taxing and a significant enough portion of their people turned out in the streets to protest.

This shouldn't be a surprise to anyone unless you're unaware that China's health care infrastructure is actually incredibly fragile and that preventing it from being rapidly overwhelmed was the real reason for the Zero COVID policy. Unfortunately you can't do that forever especially if your governmental legitimacy is based upon continuous rapid economic growth and Zero COVID strangles that growth.