southwest uses a single type of plane that cuts down on maintenance and training expenses.
southwest does not have a bloated corporate organization like many airlines.
southwest's employees get paid less than the traditional carriers.
southwest and jet blue have brand new planes that require much less maintenance. Some of the old loe cost carriers like ValueJet bought old planes that killed them in maintenance costs. FAA has strict rules about replacing parts after a plane operates for a certain number of hours.
This means that a plane arrives, unloads, loads up again with paying passengers and then takes off again. Jet Blue's leather seats are fast to clean which means fast turn around time. planes don't bring in revenue when they are parked at the gate.