Probably because they can. I have a very small and inconsequential site and I block all access from IP addresses associated with China. If I don't, it is hit constantly with attempts to sign in - hundreds per second. If I block one IP address, the attack comes from another IP address. So they are all blocked. I block many Russian IP address ranges for the same reason. Attacks still come from Turkey, Brazil, Ukraine (mainly the east) but are less coordinated.
Maybe the attacks from China are with a purpose. If the response of sites in the west is to block IP addresses associated with the middle kingdom the PRC doesn't have to do any censorship because we are doing it for them.