I live in Kansas City. From here, Southwest has a ton more non-stop destinations then the legacies, even though KC is not a hub or even a focus city for SWA. On the few routes that the legacies compete on, it comes down to a choice between a flight on a 737 vs a RJ-175.

That's right, on flights to the west coast KC legacy customers get stuck in regional jets. As a passenger I cant stand regionals for longer than 2 hours, let alone a flight halfway across the US. Southwest 737s have more legroom and overall comfier seats than RJs so I will choose Southwest every time. Factor in the free bags, the wifi, the flexible ticket options, and 4x more destinations than anyone else and its a no brainer why SWA is big in Kansas City.

Maybe it would be different if I lived somewhere else, but SWA appears to have the upper hand in the big hubs too like LA and Chicago.