A lot of it is because they suck at technology, software, and marketing. You may recall that when Apple first announced Apple Pay, a bunch of banks were worried this would lock them out of the loop, and they announced, to no fanfare whatsoever, a competitor called Isis (lol, oops), quickly renamed to CurrentC. It went nowhere and was killed with nobody noticing a few years later.

In the banks' defense, they have to work with absolutely archaic infrastructure like ACH, so maybe once the considerably-improved FedNow takes over they'll have a better shot at building something people actually want.