I expect the locked door was a major factor in the Helios, I believe it was considered likely that the cabin crew had spent their time trying to open the door. They probably forgot the code. I find a lot of cabin crew forget the code under normal circumstances, never mind in a serious emergency situation.
I don't see the need to have a door code anyway, the deny function should be satisfactory to prevent entry unless the crew are incapasitated. Then they wouldn't want to deny.