Human Factors
As a now retired "shrink" with many years working with the aeronautical industry I have found many of the posts on this thread very informative. Several have homed in on what is probably the real cause of this tragic crash - human failure of some sort. Who, how, what, when, etc we may never know but the cockpit voice recorder should give us some excellent clues and coupled to the data recorder hopefully something we can all learn from.
But for the Grace of God, etc.