One of the main reasons that so many passengers survived this was down to the Asiana cabin crew who deserve much more credit for their response and heroic actions.
From accounts so far they dealt with two slides inflating inside the cabin, initiated the evacuation themselves, cut numerous people free from their seatbelts using knifes thrown up by first responders on the ground, deflated the defective slides using the crash axe, and carried a number of passengers out on their shoulders.