My wife worked for one of the largest banks in the US for over 25 years. The last time I heard about a desktop count, there were over 150,000 people using Windows just in the US and more all over the world. There are development teams working to write proprietary software tools for these desktops and help desk people supporting the desktops.
Now imagine plopping a Linux desktop onto these machines. Suddenly there is no support; the dev teams have to be replaced/retrained because they don't write code for Linux; the help desk workers have to be replaced/retrained because they don't know Linux. The desktop user is confronted with an unfamiliar environment with unfamiliar tools and applications, and since the tools they were using are no longer available, they cannot get any work done. An enormous amount of the software being used depends on Active Directory for security and routing--all gone.
I'm not saying that replacing the Windows desktop with a Linux desktop can't be done in this instance. It just can't be done without a huge outlay of money, time, loss of work, and severe inconvenience to the bank's customers.