They did, China for one have completely shut off cryptocurrency usage within their country AFAIK, and everyone who will be found mining or generating cryptocurrencies in their households are subjected to prison or other grave punishments. So yes, they are very much capable of banning usage of bitcoin. Although I must agree with your statement, they need sheer luck and a ton of money and support from all the other countries in order to successfully shut off bitcoin, which is certainly not gonna happen, not when measures are being taken to regulate its usage anyway.