I think because bitcoins can not be controled by government and it can kill fiat money, if all of people on countries use bitcoins for payment method in their real life, the fiat money will be useless and it is mean the government can not become regulator and can not take taxes.
i think its true, maybe government afraid that in someday, people will not trust the government to protect fiat from new currency like bitcoin. the government wants to make sure that fiat still safe to be use for their citizen and make a limit for bitcoin and preventing bitcoin used many people.
government didn't fear on bitcoin can treat their national currency
but they are fear about people who can store the wealth in bitcoin without get taxed
also bitcoin suspected as a payment gateaway for illegal markets
that's why in my opinion there is so many country ban bitcoin.