Regen doesn't work in neutral because it requires the motor to be engaged, so that it can act like a generator. The single speed reduction gear "transmission" in the LEAF is what disengages the motor from the rest of the powertrain when you put the car into neutral.
So, when you put the car in neutral, there's no longer a mechanical connection between the motor and the rest of the drivetrain. In other words, there's no rotational force to turn the rotor of the motor and therefore no power regeneration.
https://www.youtube.com/watch?v=BhOEoXf ... P9LaU2wlbA