A few points....
Most US carriers have a lot of legacy costs and aircraft. Some of their foreign competition is state-owned or state-aided. It can be harder to compete with carriers that don't have the same need to be self-sufficient.
The US carriers are catering to their customers. Americans want cheap transportation for the most part and that is ok. I know what I'm about to say will offend some people, and we have our share of disagreeable travellers as well, but we tend to be a little less fussy about air travel.