Well, being an exorcist isn't like being a soldier, or a thief.  You have a very specific set of skills at your disposal, which can only be used for a far smaller number of purposes than soldiers or thieves.  For that reason, to my thinking, experienced exorcists find regions where there's a steady stream of work, to keep them afloat.  Given that he's sort of got an area established, he's not about to leave it unless there's significant gain involved.