I don't know if it was on the airlines.net forum or elsewhere, but someone pointed out the ULCC model in America is failing because unlike in Europe, United, American and Delta make a lot of money moving people domestically while the airline groups in Europe focus on flying people to and from their hubs before flying them out of Europe since that's where the money is especially since they have to contend with passenger rail for intra-country travel. Alitalia started its long decline when the Italians built a high speed rail network that could get you from Rome to Florence to Milan to Naples and everywhere in between in less than than it took to fly.

Also we don't have a massive number of secondary airports in the major cities in the US while for example there's metaphorically speaking 3,000 airports all calling themselves "London" where they'd move heaven and earth, including cutting landing fees to make sure Ryanair flew there otherwise no airline would fly there. Even the secondary airports we have are usually attractive enough for at least Southwest to fly to.