Well, why would banks replace the system which allows them to charge fees for every other interaction with their services? A blockain solution would allow multiple different banks (and, possibly, even regular people) to access the data with no middlemen, and, therefore, no fees. Or, well, no fees that directly end up in the bank's pockets as profit, that is.

Getting rid of that is bad for business. So, unless something magical happens and the EU, for example, pass a law requiring the banks to switch to a more de-centralized, more fair system, it's not going to happen.