Because once bitcoin and cryptos will step on the spotlight, banks will surely get nothing from charges they usually get on every transaction made on their banks like money transfer to cross border or country. You can think of it like what eMail did to the postal business in the advent of technology.
Big banks will be really nervous and worried in connection with the rising global acceptance of bitcoin. They fear that bitcoin is decentralized and people want it. Banks consider bitcoin as their rival and have become the bankers' worst nightmare.