The Economist's Gulliver blog explains:
America is a big country, and its airlines have to focus most of their attention on domestic flights. In 2014, Delta flew more passengers than any other in the world, but less than one-fifth of them were on international routes. Despite recent consolidation, there is still much competition at home: on top of the big four, customers can sometimes choose among JetBlue, Spirit, Frontier, Alaska, Hawaiian, Virgin America and others. And flyers have made it clear that they have one priority far above all others: price. The result, as one airline industry veteran told Fast Company, is that flights resemble “a Greyhound [bus] with wings”. When flying halfway across the country for $70, can flyers really expect better amenities than on a long-haul bus making the same trip?
That pressure is not felt by, say, Singapore Airlines, the world’s third-best carrier, according to Skytrax. It has no domestic routes to worry about; the entire country is less than 300 square miles.
If America wants better airlines, in other words, the answer is simple. First it must take the carriers into state control and squash the unions. Then it must persuade the world to forget its sexist ways and encourage those at home to go on a diet. Finally it should reduce in size and move a bit closer to Asia. Easy.
So history, geography, and culture all play roles? Who'd have thought.