Because Bitcoin is essentially a weakening of government power. But on the other hand, it is difficult for the government to fundamentally ban Bitcoin.
you cant ban bitcoin.. just like a bottle of 1920's beer has no brain or eyes to read the law for a bottle to realise it is banned
what governments can do is ban its citizens from using it.
just like prohibition in 1920's it did not ban beer. it banned people from drinking beer.
governments can control their currency by ensuring people are paid wages in only fiat. and pay taxes in only fiat. thus making fiat dependant on peoples income
this is why bitcoin is no threat to developed countries.
however small undeveloped countries like el salvador that doesnt even have its own fiat. is more free to try new things and detach itself from the dependance on the US dollar.
dont think of it as a binary option. bitcoin to replace all fiat or none
think of it as bitcoin is a second choice. take it or leave it