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 there is no real reason to make it illegal. Because for now, far from threatening banks and government. If so, the government should be able to regulate its use, not prohibit it completely. Different countries have different views on this, and I doubt the rest of the world would agree to ban something like this. Because Bitcoin is the future technology, No one can refuse the technology.