I believe it used to be the case that a very large amount of processing was done when the bank wasn't "open". This was done after 3pm, but before 5pm, so the employees could go home. Any customer transactions done after 3pm were queued to be dealt with the following day.

Banks used to close their doors at 3pm - I remember this distinctly when I had my first job. Yes, things are different now, and the processing done is less batch-like, but there are still large amounts of regulation that need to be adhered to, and sometimes not taking transactions is the most efficient way to do that.

It's only sometimes the fault of the bank.