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.
yeah potential in bitcoin might make fiat become irreplaceable. perhaps it is thought by governments which do banned, they think that bitcoin is growing very fast in their country and it threatened them