It's a distributed trust network, right? Why would banks that survive on trust want that distributed?
Apparently, they don't. Each institution is building its own, private blockchain to stay buzzword compliant, not because it makes any technical sense.
It's like hearing about the Internet for the first time, and proceeding to build a private, closed version -- which really happened several times, but eventually people realized that the whole point about the Internet is not being private or closed.