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 ?
They can't make it illegal because they found out that they also need the technology. Blockchain technology have a very wide coverage in terms of its usefulness. Implementation of blockchain in a currency is just one of them. You can use blockchain in any industry and that is the fact that they want you to know.