The USG, as powerful and full of unlimited hubris as they are, cannot simply make bitcoin illegal carte blanche, although I'm sure many of them would like to do that if they could get away with it.
They can make anything illegal, so long as it stands up to challenges in court, and even then they can make it illegal for a time. There's a difference between whether something can be made illegal and whether something is enforceable.
The thing is that just if they do make it illegal, that will scare a great number of potential users from using it. The current Bitcoin community demographics do not exactly reflect the economic demographics of any country, and for Bitcoin to achieve widespread adoption, it needs to be seen as legitimate by the market participants who see something being illegal as making it illegitimate in their minds.