They already did. From my memory, there is several major banks, including Central European Bank, which clearly said to people that it was dangerous to invest in Bitcoin, and that they should focus on their investment plans with ridiculously low yields.
Banks will always try to scare people away from it because there is no way for them to profit from it. They try to make it sound scary and super risky, but I would assume that anyone who actually does the research would see through their warning pretty easily.