Southwest likes to control its own baggage handlers - it pays them well, but works them hard. In order to employ and pay a full time crew of baggage handlers that will unload an arriving plane and load the same plane for departure all within 45-50 minutes, so it can achieve a turnaround time of an hour or less, 8 turnarounds per day, 5 days a week, would account for a tad less than 35 hours a week. Sure, Southwest could just use the baggage handlers employed and paid by the Airport, and not worry about how many turnarounds it did per day or per week, but then it loses control over those employees, and can't prevent them from taking their sweet time to turn a plane around.
BTW, the same logic applies to the Ground Crew. While the baggage handlers are unloading and reloading the plane for turnaround, the Ground Crew is cleaning the cabin, restocking the galley, inspecting the plane, refueling, cleaning the windshield, etc., everything necessary to prepare for a new bunch of passengers and another takeoff. In a domestic airport used by Southwest, it employs its own Ground Crew. Without that minimum number of landing-takeoff cycles per day, Southwest couldn't do that at Cozumel.