Had the pleasure of listening to Herb Kelleher SW founder at the IATA Annual Conference in Washington recently. The most entertaining and irreverent speaker.
SW have been successful for a number of reasons
1. Looking after their staff and great working environment
2. One type of aircraft B737
3. Selected popular routes and not tied to hub and spoke that can mean multiple aircraft changes (this is also why corporate aviation is so strong in the US as business people want to go from A to B direct not by C D and E.
4. Use of secondary airports near the major cities.
Interestingly, The CEO of Delta followed Herb and I must say was also impressive but admitted the hub and Spoke had its down side.