During a storm, there are many things that go on at airports that affect why certain airlines cancel a lot of flights and other fly "normally". A couple of years ago, during one of the major snowstorms of the northeast, one of the legacy airlines cancelled my flight and said I had to wait 3 days before I could fly out. I called Southwest and booked a last minute full-fare ticket to fly out. The storm had stopped and there was sunny clear skys and the runways were clear, but I imagine it had something to do with staffing or aircraft availability.