Why does southwest have a standardized fleet of boeing 737's?

Southwest airlines is a major U. S airline headquartered in Texas, and the world's largest low cost carrier. One of the distinctive capabilities of Southwest Airlines that makes it different is that they operate one type aircraft which is the Boeing 737, a short-medium range twinjet narrow-body airliner which delivers superior reliability, and fuel efficient. It is low maintenance and operating costs a competitive edge.