According to an analyst that CBS talked to https://www.cbsnews.com/news/southwest-flight-cancellations-... Southwest was hit harder because of they way they organize their routes. Not having "hubs" means they can't create alternate routes quickly. So the delays in Florida cascaded back, stranding planes and crews as well as passengers. He also says they'd already scheduled more flights than they can handle, but I don't know exactly what that means.