While the United States is home to the world’s three or four largest airlines (depending on the metric), it doesn’t operate the largest passenger aircraft. No US-based airline ordered the Airbus A380 Superjumbo, even though Pan Am had been the first to fly the Boeing 747-100 Jumbo in 1970. United Airlines operates the second-largest fleet of widebody aircraft after Emirates, and yet it didn’t order the Airbus A380. But why?