Fear, panic and training. I’ve frozen up plenty of times and not reacted when the fecal fan interaction was in progress.
Also, because of angle of attack protections in the Airbus, you can hold full aft stick and the aircraft will supposedly not exceed the critical angle of attack in normal law. Below thrust reduction altitude, usually 1000 feet, the procedure for Unreliable Speed/ADR Check procedures is to select TOGA power and 15 degrees nose up. At altitude it’s 5 degrees and climb power.
Also, and I haven’t read the whole thing in a while, but the plane was probably not in Normal law meaning the angle of attack protection was inoperative. In fact, it was likely in abnormal attitude law meaning the flight controls were responding to direct pilot inputs with no protections.
Lastly, the other FO and CA were unaware that the flying FO was holding full aft stick until he mentioned it very late into the situation. There is no visual or tactile indication of that fact with the other stick unless both are moved simultaneously.