Fatigue
Does anyone know when the crew started their duty?
Somewhere amongst the posts it is mentioned that the aircraft had operated a postal flight during the night. If the same crew had been operating then, they would have been exceedingly tired, regardless of whatever pre-flight rest they achieved, at the time of the accident.
Having flown through the night for several years I and my colleagues were only too aware of the degradation in our performance at the end of a long duty.