I can think of 2 possibilities offhand:
(1) Receptors for mitogenic growth factors are not expressed.
Or the signaling pathways that are triggered by growth factors
lead to different 'ends'. For example, NGF is a survival
factor for neurons (through the trk receptor), but trk is
mitogenic when expressed in other cells types.
(2) The structural components of neurons (e.g., bundles of
microtubles, etc.) will not permit neurons to attain a shape
that permits cell division (that is , they can't 'round up').
Other suggestions ?