the main reason is that cryptocurrencies are decentralized and they can not control cryptocurrenies. they don't want something that they can not keep it in their hand.
Agreed because all governments want to control all things related to finance and their economies. Sometimes uncontrolled currency can affect their economy when it go out of hand and this is one of the reasons of banning it. Cryptocurrencies can't be control by any government but they can impose regulation on any centralized cryptocurrency exchangers instead to monitor the flow of the coins.