In normal flight, pull back stick == plane climbs up.
When plane plunges down, instinctive reaction == pull back stick (to make plane go back up).
All pilots do this almost sub-consciously. Feel plane sink a little? Pull back the stick a little to compensate. In 99.99999% of the cases this works out for the pilot, i.e., in normal flight with sufficient power and nominal AoA. (And especially with a working autothrottle).
Of course we all "know" that this "doesn't work" during a stall -- or even near a stall (area of
reverse command).
Thus the problem with surprise stalls is that the pilot must overcome his/her instinctive reaction (which until this point has worked 100% of the time) and do the complete opposite: push the stick down when the plane is already going down.
To summarize:
- Pushing the stick down is an "intellectual" reaction. It requires knowledge that the plane is in a stall, and recall of the correct stall recovery procedure.
- Pulling the stick back is a "instinctive" reaction. It is based on "muscle memory" from thousands of hours of normal operations.
There is a human-factor safety theory that says that if you have to "remember" to do
something different (opposite) during an emergency vs. normal ops, then you will always have a high risk of failures.
Training does help, but only to a certain extent. Training happens at most a few times a year. Yet pilots "pull the stick back to climb" on every single flight. That's "training" as well (from a neuroscience perspective).
I'm sure every single pilot familiar with AF447 and QZ8501 has concluded that "this can NEVER happen to me!" but the truth is, when you are startled and confused, it's
hard to fight instinct.
Imagine it's year 2100. AirBoeingBus 78320-ER somehow gets into a stall and starts losing altitude. The Pilot, last of her kind, pulls back the stick, to go back up. Autopilot-G (for George) recognizes the stall, automatically lowers the nose to reduce AoA, recovers from the stall, and then pitches up until the plane climbs -- as commanded. In this case, the pilot does nothing different between normal and stall condition. From a pure human-factors perspective, this is the preferable system behavior.
Edit: to add, even in the non-stall case, when a plane is rolled with nose down attitude, the
instinctive (and incorrect) upset recovery action is for the pilot to simultaneously apply opposite roll
and pull back on the stick. This is in part why UPRT training is being mandated.