Because they are the country who cares about the future and thus are trying to control the situation because it is better that way than for it to get out of control and then they will hit big wall. If you look for instance US is also closely watching what's happening and EU. China is aware that this whole thing can disrupt the money system that was built before and questions such as how you tax it, who to trust etc. should be asked. To my understanding China is mostly negative towards ICOs because of the way they are funded and because there are scams related to it.