This is one reason why the government fears and interferes in the worldwide adoption of bitcoin. They wanted to mingle with its every transaction; it's just that they can't figure out yet!
They want to know everything about the money flow. For good reasons (terorism), for possibly good reasons (find tax evasion), for not so great reasons (general surveillance)...
With Bitcoin this is sometimes possible already, with the help from Exchanges (KYC). The only possible barrier is the mixers, but it may not be as big barrier as some would think.
If bitcoin goes mainstream without the interference of the government, living up to its very principle of decentralization, then how could the government earn?
Governments will still earn in the same way as today, from taxes. Bitcoin may help people avoid taxes, but it's not in such big amounts it would hurt the governments...
They will obviously lose power over the economy of its nation. So apparently, this is the main reason why they keep on delaying
Bitcoin will not replace local currencies. It will be just another coin in the world. A very small number of countries may adopt it, but even that is hard to believe. So no, governments will lose nothing.
The ones losing are the commercial banks. They will lose some parts of the market share and they'll have to make some of their services much cheaper. And banks have money for lobby. There is where the problems start...