With bitcoin, people keep the money in their personal wallets were government have no access. They cannot know how much wealth anyone has and hence they can't collect taxes. Without the tax money it will be difficult for the government to manage the country. This is one of the reason why govt does not want people to invest in bitcoins. There are other disadvantages too like with bitcoin money laundering is easy.
But there are good aspects of bitcoin too. If an underdeveloped country invest in bitcoin and in few years when the bitcoin prices 2x-3x, then can sell and get rid of their loans.