Southwest Airlines was the grinch that stole this year’s Christmas for travelers across the country. The airline has canceled more than 15,000 flights since Dec. 22, stranding passengers and crew for days. Even worse were the horror stories of passengers unable to reach customer assistance, let alone rebook their flights, and sleeping en masse at airports. While every other U.S. airline managed to get back on track shortly after the rough Christmas week weather, Southwest has only seen its problems escalate: It accounted for more than 90 percent of Wednesday’s flight cancellations by U.S. airlines and about 99 percent of Thursday’s — a full week after the winter storm initially hit the Midwest. This is no longer about weather affecting flights. It’s about basic competence, and Southwest deserves a failing grade.