The US may be huge. But regions within the US are not huge. There are at least a half-dozen regions in which high speed trains would make as much, if not more sense than they do in Europe if judged by population density and distances.
Because the environmental impact of trains is, while controversial, unquestionably less than that of flying.
Because trains should be moving at an average of 140mph or so, which is more than twice that of a bus.
Because most airports don't connect you to the entire globe in a single hop, so you may as well take the train to the existing one that does.