Most domestic airlines have well planned contingency plans for northers weather like snowstorms. Based on routes these planes tend to sit planes out in the warm aprons of MCO etc. til they can get back into some semblance of route planning. Rarely do you see the likes of the same airline flying in the last flight of the day to a northern city and get the plane cancelled and stuck there.
I suspect that their experience with how to best recover schedules is easiest when they park the planes at these same specific southern airports and return them to service against a smooth and tried plan.
In other words an extra day on the ground is a lot easier to handle when they resume service than having those planes and crew scattered somewhere else.