China has not banned cryptocurrency. They just banned ICO and exchanges. Peer to Peer trading is still available and actually quite popular in China. I don't think there is an effective way for government to ban bitcoin anyway.
Right, government cant ban bitcoin easily because they need this technology. Government may regulate bitcoin but for them get some taxes, government will find ways just to control bitcoin that can never happen.
Several other reasons why some government banning bitcoin is try to avoid illegal use by people.It can be used to buy weapon, drug, terrorist funding and money laundry. These transactions are difficult to be traced by the government.It is not expected also that local currency is influenced and become unstable.