I believe China's gov't is more interested in the money invested in the current real estate problems (see Evergrande) not leaving the country through coin-exchanges. They want the money to stay invested in the country, even if it's not in the real estate companies. (Edit: also, the power problems, as stated above)
But whatever the reasons are, I'm good with the ban. I hope more countries stop allowing conversion to local currency (like the US), and then all of the power and semiconductor resources can go into more productive uses than cryptocoin speculation & ransomware payment-enabling. If I never read another story of how a coal-burning power plant is being "repurposed" to supply power for mining, then that's a great win environmentally. Getting the GPUs being bought up by miners back into the hands of the long-time users of them - gamers - would be nice. But even more important, all of the industries who can't get enough processors - more semiconductor resources should be available to them. A worldwide ban would be fine with me - only ones who lose are the ones who were speculating (gambling) in uncertain futures.