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.
Bitcoin will be supported by the rest of countries because biotin is fast growing currency in world and as some countries accepted bitcoin the rest of countries will also accept bitcoin and the countries who accepted bitcoin they also take their time to join because may be they want to see the result of bitcoin in those countries who have accepted bitcoin its such a currency which can reduced unemployment and also it can help them in their economy.