Competition on high volume routes plus larger aircraft on those routes to amortize the costs. This is exactly what happened with trains. High volume routes saw

Competition on high volume routes plus larger aircraft on those routes to amortize the costs. This is exactly what happened with trains. High volume routes saw a lot of competition and were very cheap and local routes were very expensive. So I can often fly Denver to London cheaper than Denver to Durango, CO. There's more parallels around regulating trains and how the train companies were eventually bankrupted and taken over, with the inverse happening around when Southwest was new.