The governments can't control Bitcoin because it is decentralized. It wasn't made in favour of a centralized system where one person or authority can have full power and control over its transaction. The farthest the governments can go is to impose regulations on Bitcoin transactions especially on the exchange sites.
Even if the government should put regulation in place, like you have said it’s only going to be effective on exchanges, do you mean only centralized exchanges or the both types of exchanges?
Because, the way I see it the government can only be able to regulate the transactions that are taking place on centralized exchanges, decentralized exchanges are something else and I don’t believe that the government can have any kind of control over them since they are decentralized. And also when people are doing transactions peer-to-peer the government won’t be able to keep an eye on it.