I am a little puzzled that we speculators have not included mechanical/system failure in our lists of possible causes
A common description of the evidence we have seen so far indicates insufficient power/energy to complete the maneuvers observed immediately before the accident. Further observations are that there is no aural evidence of engines spooling up.
Possible failures that could account for this include fuel contamination, fuel system failure and throttle/power control system breakage or failure.
Of course these would be rare but then again, accidents are rare!