For me, I think it is because bitcoin users are unknown, unless they tell themselves. And because of that, government will find it hard to let bitcoin users pay for their taxes, unless they are honest enough. Aside from honesty, I could not find ways on how to determine how much a person own crypto currencies.
Well, yes. Bitcoin users are unknown, unless they tell themselves. Bitcoins are unknown because Bitcoin is anonymous. It is anonymous because a bitcoin user can tranfer and transport a lot of money in just some clicks. It also cause Money Laundering. Bitcoin is not controlled by the government, so that, bitcoin don't pay taxes in every transactions and it is running in the world with anonymity. That's why bitcoin is not supported in some countries.
Governments can still impose taxes on crypto by their gateways. But what really bothers them are possible money laundering. Bitcoin is already popular in deep web which is of course not cool on governments. Terrorism and countries like North Korea are also possibilities why these countries will never tolerate and as much as possible will try to control this technology.