The cynic in me winces when thinking of the mess all the banks and interested companies would make trying to get together and build a payment standard. Just look at the terrible mess of stuff like OFX.
Ah, a standard protocol where nobody implements the standard in the same way. As someone who has been working with FIX for few months, I could totally feel the pain.
Also it might not be necessary for the involved parties to get together. They could release different protocols. And then the lesser used will be forgotten (like gopher is forgotten today) and the more frequent used will survive (like http has survived).
Well, the point of those payment systems is to make money out of fees and basically control the whole system. You cannot do that with decentralized payment methods.