The procedure to recover the plane was already released but not followed correctly by the Ethiopian pilots, the Co-pilot was woefully inexperienced, leaving the Captain task saturated.
The first officer, Ahmednur Mohammed, aged 25, had 361 flight hours logged, including 207 hours on the Boeing 737
NTSB findings:
Appropriate crew management of the event, per the procedures that existed at the time, would have allowed the crew to recover the airplane even when faced with the uncommanded nose-down inputs.
During the accident flight, the flight crew did not make appropriate use of the associated applicable procedures on which he had received training in the preceding months.
The Captain's attempts to engage AP was in contradiction with the Approach to Stall or Stall Recovery maneuver check list, which was expected to be applied in reaction to the stick shaker activation.
Degradation of the CRM which started immediately after the AOA vane failure and which didn't help the crew take the necessary actions to keep the plane under control although they had received an adequate recurrent training on situations that occurred in the accident flight.
The preliminary report asserts that the thrust remained at takeoff setting (94% N1) and the throttles did not move for the entire flight.
The full throttle setting made trimming near impossible.
Pilots have demonstrated in simulators that the trim wheels cannot be moved in severe mis-trim conditions combined with a high airspeed. As the pilots on Flight 302 pulled on the yoke to raise the nose, the aerodynamic forces on the tail's elevator would create an opposing force on the stabilizer trim jackscrew that would prevent the pilots from moving the trim wheel by hand.
https://en.wikipedia.org/wiki/Ethiopian_Airlines_Flight_302#Investigation
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