That was a missed checklist item IIRC where the ground engineer had turned off the pressurisation while trying to debug a pressure leak (ironically), the pilot was supposed to check this in a couple of lists, they did but I think they glossed over it. Plus they might have ignored some inflight warnings. This is also the one where a learning pilot (also scuba diver, probably able to handle the lack of oxygen better) tried to save it but was too late.

That one haunts my nightmares.