Because it's cheap.
I don't think it's necessarily that Southwest is the best, but that's generally where people can get the best fare.
There are trade-offs though. There's no assigned seating, and many don't like the cattle-call style boarding.
But that's all personal opinion.
Personally, I love that Southwest has a nonstop flight out of Buffalo to Orlando, and if the plane does stop, it's usually in Baltimore, and I don't have to de-plane.