“Unbanked” people are those who operate only with cash and don’t want their money tracked. If you haven’t a bank account, it’s harder for the IRS or divorce courts to get hold of the money.
I can see many people avoiding PO check cashing for just that reason. The PO would keep electronic records accessible to courts and IRS, and would eventually be called upon to garnish wages — AFTER all the other competitors had been driven out of business.