Answer is quite simple. Steadiness.
Both economy and politics.
People in China always follow the crowd. When there's a asset being hot and highly profitale (or even looks look profitable), crowds with unthoughtful people will come, and pump the price extreamly higher. Imagine 1.4 billion people invest bitcoin or shitcoins, and imagine the 50% dumping in Jan, how many people will jump from the top floor? And simply some steadiness about finance linquidity, foreign exchane loss, wow^^
Those will finally come to the steadiness of politics. For a despotism, what they need is steadiness, people working hard for the living, no troubles, no arguement, no unsatisfactory.
That's all.
It seems to me that the cause of the other. I do not believe that the Chinese government cares about is to protect its citizens. In many villages of China now there is no elementary medical service. There is no reason that the use of cryptocurrencies makes people independent of the monetary policy of the state. They are free in their actions. These people are hard to manage.