No-one was prepared to host it in their premises.
PO Ltd tried for over 2 years by going around speaking to shop-keepers and owners / managers in the town and advertising online to find premises for a three till PO service, then a 2 till service and then gave up and very specifically approached McColls again and agreed a 1 till service.
As a town councillor until I resigned last Summer, I set up and chaired a town council working group which looked at the possibility of the town council setting up and running a main PO in the town centre. Unfortunately, most town councillors were not prepared to support the sort of financial subsidy it would have needed, which would have been roughly equivalent to the council's current most expensive ongoing annual project - running The Laverton building, which loses about £30,000 ish a year (including "rent" received for the town council's own offices).
This was part of the reason I resigned off the town council. In reality I was deeply fed up with the role of being a town councillor anyway and just ran out of reasons to stay and was overwhelmed with reasons to use my time doing other things.