I think because they found that some people use Bitcoin in illegal transactions, they fear that fiat currencies will no longer apply to people and as a means of payment are not recognized and not protected by law, and credit institutions are not permitted to use it as currency or payment .
This is not illegal it's just decentralized by them which they are the one who discriminate and lookdown bitcoin because they already have what they supported which is bank, only governments are having negative thoughts in crypto but most of people are likely to involve due to the opportunities. If they will banned crypto then probably there are other ways to transact because there are also some governments involve in crypto,they are also produces coins or programs.