Remember Costa Concordia and that Korean newlywed couple saved from their cabin the day after the grounding?
They were told to "go to their cabin". So they did.
So did the children on the Sewol when they were told to "stay put". They were used to obey orders, not to question them, even against their better judgement.
Situation on the Sewol did NOT escalate incredibly quickly after the initial incident. Survivors said they were waiting "30 to 40 minutes". Until the moment the "rolling cargo" on vehicle deck shifted, and then it was too late for anything.
Safety drills involving passengers on Korean ferries do not exist (I've taken Korea-Japan ferries dozens of times). They do, however, broadcast over PA that "passenger safety is of our main concern", but it is up to passengers themselves to familiarize themselves with muster stations/safety equipment.
Anyway, we are still to learn about the cause of the sinking, but the loss of life was entirely preventable, IMHO.