In one of the introductions to the series (I recall it from the Revised and Expanded edition of The Gunslinger), King mentioned that he liked the marvelous sense of dislocation seen in some of the old spaghetti Westerns. Maybe he had that in mind and jacked it up to 19 when he was figuring out what to do with Maerlyn.

Its possible that Maerlyn had a change of heart after doing his work on Walter Paddick and went into seclusion as pennance for his deeds. Maybe after Walter obtained Black 13 or the other glasses of the Wizards' Rainbow, he became to powerful for Maerlyn to deal with.