My bet will be on core banking software provider issues. I think they did try to talk to theirs and got a response like, "We can do it in 5 years, and you will have to pay XXX million to us to do it".

There was a large-ish scandal in the late 2010s when some core banking providers were delaying Zelle integrations for smaller FIs, and they started complaining as customers were demanding it and leaving the larger competitors with it.

Another option can be as simple as this - they fear tech as most don't understand it. When I was working on a corporate charge card startup, my co-founder and I faced this issue many times. The craziest experience was when the bank was ok to be issuing bank for us but requested our clients to go to their branches to sign paperwork (the bank literally has maybe 5 branches in the whole USA).