Because women are obligated to work 40 hours every week just to stay afloat. Unless you find a way for young educated family to live a decent life on a single husband's income, the trend will continue.
US, Canada, Europe are demographically cooked too, but they (partially) solve this problem with immigration, that is not an option for China and the rest of Asia.
This can not be overstated. China is on the verge of the largest population collapse in human history.
By 2080 China's population will drop by 600 million. If that trend continues, by 2150 China's population will drop to 280 million.
Other asian countries such as South Korea and Japan are on a similar trajectory.