[mst]

My guess would be that a tiny percentage of accounts didn't get their notices sent, nobody complained about it because they didn't realise they were supposed to be getting them (or at least for whatever reason no such complaints were escalated to the people who would go "oh shit" upon seeing one) and some unfortunate combination of technical factors meant whatever internal monitoring existed didn't pick up the omissions.

Note that this is not to say the end result wasn't an indefensible disaster, only that disasters seldom have only a single cause and the above is my best uneducated guess at how things came together to cause this one.