My wife, son, and I just watched "The Search for Spock" and we were all wondering why David didn't regenerate back, like Spock did when he got stabbed by the Klingon. Does anyone know?
Spock's corpse was shot onto the planet while the Genesis device was still working on the planet. So Spock's cells were regenerated by the device making life on the planet.
By the time David is killed, that process had begun to break down. David had revealed the cause in a prior scene: he used protomatter, which made the initial process work, but resulted in the major instability that destroyed the planet. From the transcript
SAAVIK: It's time for total truth between us. This planet is not what you intended, or hoped for, is it?
DAVID: Not exactly.
DAVID: I used protomatter in the Genesis matrix.
SAAVIK: Protomatter. An unstable substance which every ethical scientist in the galaxy has denounced as dangerously unpredictable.
DAVID: But it was the only way to solve certain problems.
SAAVIK: So, like your father, you changed the rules.
DAVID: If I hadn't, it might have been years, ...or never!
SAAVIK: How many have paid the price for your impatience? How many have died? How much damage have you done? ...And what is yet to come?
David didn't have enough time
The later stages of Spock's regeneration resembled typical (though accelerated) postnatal development. Earlier stages may have therefore resembled embryonic development. Although the progression of the Genesis effect makes comparison difficult, it's possible that David began to regenerate similarly.
As Spock was regenerating into the form of a child, his crewmates had time to limp back to Earth in the damaged Enterprise, rest at home, settle into new jobs, devise and implement an elaborate plan to steal a heavily-armed starship, and return to Genesis. If David regenerated according to a similar schedule, then the process would have barely begun by the time the planet tore itself apart.
 In pulses, at decreasing intervals.
 Assuming Spock regenerated only once, instead of over and over and over.