A lot of what seems apparent in this case is that contractual and commercial factors weren't set up correctly - they were set up to deliver predictable prices (loved by public sector clients), but not necessarily to deliver good outcomes.
An example - it appeared much of the rush to ship the point of sale terminals was to get through customer acceptance (and presumably the payment milestone), despite scope creep and quality issues. And there was a cost for PO to access audit logs, and limits on capacity of these logs which could be handled. Presumably this delivered a lower headline price for an accountant negotiating the price down, but it ultimately made a poorer solution that wasn't for for purpose.
It seems like (from what I've seen of the evidence) nobody internally in PO really had full understanding and ownership of the project and they'd outsourced that (but kept the suppliers tightly commercially managed, creating the incentives for shipping poor quality code, rather than spending the time to polish it, as some had tried to do in the development team).
Some interesting evidence here in the inquiry on software development practices and low competency and quality of code - https://postofficeinquiry.dracos.co.uk/phase-2/2022-11-16/#d...