The problem with government not stopping bitcoin is not that they cannot its because they don't want to. Government of countries can be so radical that anything they want can be forced down people throat depending on how far they are willing to go. Although, when it comes to bitcoin, they might not achieve 100% compliance but they can achieve some maximum result and the rest of people would stay away because of fear that they might be punished. I have seen a country shut down its internet because they want to ensure certain action and ordinary citizens in that country could not access internet in within that period. The reason why government does not want to go that route is one, the commercial value of those involved in bitcoin or its related activities is not enough to attract that kind of enforcement and secondly because they don't want to lose out of what is to take sway in the nearest future.