Could it be possible that the pilots thought they were in a nose-down attitude?
They probably had no external visual reference, and with the deceleration of the plane, even with the slight nose-up attitude, it would have felt like they were in a nose-down attitude. Faced with an altimeter spinning backwards, and thinking that they were in a nose-down attitude, the last thing on their minds would have been to push the nose down any further...
Sorry if this is a silly suggestion, but as a non-pilot I am struggling to understand how three pilots could not figure out that their plane was stalled.