It is a little like modern aircraft disasters. A technical failure or combination of failures, perhaps even minor ones, combined with bad/wrong immediate management of the situation when the fault is detected, compounded by some matter of policy. In this case a refusal to acknowledge at the highest level that the system could possibly be wrong. "Clearly it must be the sub-postmasters stealing".

Edit: I would also add that successive governments have shamelessly dragged their feet over this.