Because the actor who plays him, Jay Mohr left to play the lead in the show "Gay Unmarried".. also a CBS show.
.. They very much left the door open so that Professor Payne could also come back & at least star in a few eppy's if nothing else. And I know that Jay left the show in good terms w/ every1 & everything like that. Soo I would say it is def a possibility that he could return by the series finale of GW. ;D
