Because the stall warning is inhibited below 80 knots (IIRC) indicated as no one would ever try to fly that slow. That prevents nuisance warnings on the ground when the aoa vane droops due to low/zero airflow.
The only time the back stick was released the speed increased above the inhibition threshold causing the stall warning to restart. The PF’s response was to pull full back stick again which slowed the aircraft back below the threshold causing the stall warning to stop again.