As far as I know, he just chose to move on to other things. Also, Costello was brought to the show initially to provide the production expertise that Dan Curtis did not have. Two years into the run, Curtis had the experience to play a much more hands-on role, which may also have altered the contribution of a producer. For instance, having spoken briefly to Peter Miner, he recalls being specifically being employed to oversee the day-to-day running and admin side of production, and took little or no creative responsibility.
