I'm wondering this because is such a real threat to bank system and we all know how banks and governments are tightly linked !
BTC could make people more autonomous, self-resilient, being their own bank. You can use it like cash, making your transaction almost untraceable (or untraceable using e.g. monero or dash or laundry systems) : this is governments worst nightmare, isn't it ?
I think the thing that make government don't make BTC illegal Worldwide because of they believe that it is a good Technology. Also, the thing that make it illegal is not because of the bitcoin, but the way you use bitcoin as an user. When you use it to do Money loundrying or buy some drugs and so on, it means that you are that doing something illegal, not the bitcoin.