Programmed for Murder - S8-E18
Revealing mistake: In the outside shot of the hospital, just before Mort discusses his cough with Doc Hazlet, you can see a sign saying the hospital is located in Ohio, not in Maine. This isn't the only episode where this exact mistake is made.
Answer: The storyline was that Jessica was teaching classes in New York. However, after so many seasons and as ratings declined, it was probably to freshen up the series with a different locale, introduce new characters, and have a greater variety of situations in which Jessica could solve murders. It was already unbelievable that so many people could be murdered in the small town of Cabot Cove. Being in a large city like New York makes it a tiny bit more plausible.
raywest ★