It's not about the government is unable to put a tax on it, they can actually do that in some ways, I believe the reason is the government until now can not give their final decision for crypto currency regulations but it is still under observation on how this platform be in use effectively.
In our country, bitcoin is not regulated but it is being put into observation because the users here are increasing. The bad news about it is also growing since it is used for fraudulent actions so I am afraid that it will get banned in here also. I only hope that they are considering the advantage and benefits that cryptos can give.