Since the cockpit door on any commercial airliner - including the 737 series - is not an airlock, when the cockpit lost oxygen and/or pressure, so did the main cabin. So everyone on the plane was incapacitated or deceased and nobody could have done anything even if the cockpit was wide open and every passenger was a trained and certified 737 pilot.

Not to mention, a passenger text-messaged their cousin via SMS that one of the pilots was "blue in the face" (meaning apoxia - lack of oxygen - had set in) and that they themselves were "freezing" (likely losing feeling in their extremities as apoxia set in on themselves). Now chances are the pilot this passenger saw was probably the First Officer and they were probably in the cabin, trying to determine the issue. But that means they were aware something was wrong, but were overcome before they could react.

This is probably a similar problem to what happened to golfer Payne Stewart's private jet. The problem was so gradual that the passengers and crew never knew what happened to them until they were incapacitated.