Also interchangeability.
If you always fly one type, then any aircraft can fly any route with no issues.
If you fly more than one type with different seating capacities, you have to (1) allocate them to routes and (2) sell the seats.
If then an aircraft is sick you may not have an aircraft large enough to seat the booked passengers. You then have to get into rebooking or denied boarding compensation etc.
It just keeps things simpler.
btw, you could also include RYR with the 737-800.
All are based on the original SWA model in the USA.