Banks generally charge fees for doing anything with money, even just holding on to it. That's because the banks have created a level of trust that transactions pass smoothly and everything is recorded and accounted for correctly. Therefore, they know that Bitcoin and other decentralised cryptocurrencies allow people to trade directly with each other, cutting out the need for a middleman which, in traditional commerce, is a bank.
Banks aren't happy about being made obsolete and with help from governments they are trying to clamp down on cryptocurrencies after initially dismissing them out of hand.