On what legal grounds?
Maybe in a totalitarian communist regime bitcoin can be made illegal, but in the free world a legal case would need to be made. I have never heard one, have you?
You don't need to live in a totalitarian and communist regime to experience banning of something. In EU for example, laws have been voted to ban glyphosate (Monsanto Roundup) use for example. In some countries of the free world some gov's plan to make the use of a bicycle with helmet illegal so why not BTC ? If a government judges it as a tool for money laundering they can decide to vote a law to make its use illegal.
Bitcoin is full of risks involved, starting from the virtual presence without a physical state to the risk of hacking. When a central controlled investment scheme goes as a scam, the people complain and the government take necessary action. With bitcoin lost is lost forever one can't raise it as an issue anywhere else. This makes governments stay away from bitcoin.
Bitcoin became a risk when greedy people think that this decentralized virtual currency can be a medium or means of investment and made it to reality. The sole purpose of Bitcoin is to simplify our payment and monetary system to make lives easier but look at Bitcoin today it become an asset more than a currency what is supposed to be.
Some governments are very optimistic even they know that Bitcoin or other forms of crypto currencies are too risky and its value is very volatile they still believe that this technology will surpass its downside in the future when the majority of the countries implement rules and regulations on crypto currencies. But some government see it as a threat and fear that the money of their citizens is at risk of being stolen that's why they are banning the usage and possession of crypto currencies in their country.