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.
You can find hundreds of reasons for banning cryptocurrencies. And everyone will be just as good for governments to introduce restrictions. However, I think that a way must be found to reduce fraud and use the crypto for criminal purposes. This will significantly improve the image of cryptocurrencies and will facilitate the path to their widespread acceptance.