Because China is influential in the cryptocurrency industry with them recently holding the biggest mines in the world and with that the hash rate of bitcoin, and thus when they do something related to cryptocurrency, then be sure that it will affect the prices. Not to mention that they have a big population.
China's sphere of influence is not confined to the crypto industry, their sphere of influence can affect different industries when their economies are claimed to be very strong and inclusive in terms of mining and manufacturing industries, moreover, the mining industry is relatively public in China and the recent sealing shows that the development level of this industry is huge in China, quite a few factories have migrated. Bitcoin may lose its stability in value but this is a joy for some people as mining activity is relatively smooth during this time.