It was the strategy of Southwest to keep only a fleet of 737’s for the following reasons:
A. Maintenance and parts are abundant. So they could easily stock needed parts and save time and reduce costs.
B. Training on a 737 especially the 200 series was easy and once you have learned the plane and systems. You can move about each aircraft with familiarity.
C. AD’s. Or Airworthiness Directives issued by the FAA were easier to keep up with and to comply with.
D. The 737 could easily get in smaller airports and land and takeoff on shorter runways.
These are the main reasons for their decision. The DC9 was more expensive to operate in the day I had a friend who used to fly the 9 and he used to call it the whistling shithouse. It was loud.
Also probably a big reason to fly the B737 was Airbus was not in existence in the 60’s.
Southwest in my opinion is the best run airline in the world. I recommend a book that the founder wrote called, “Nut’s” by Herb Kelleher, the founder.
By – Donald Payseur