For one cryptocurrencies threaten the governments' monopoly on money. Having a monopoly, especially over something as essential as money, grants one power. No one likes losing power.

Second, the current status quo of the monetary system works very well for the people on top. So obviously one is hesitant about changing the status quo.

Naturally both assumptions contain their fair share of paranoia, which may or may not be warranted. So far only few governments actively battle cryptocurrencies or try to smother its market. However there seems to be a correlation between a government's urge to control the economy and money supply and the extend of their crypto regulation which leads me to the conclusion that the assumptions above at least in part reflect reality.