For the average flyer, WN offers a much superior Y product, friendlier staff, and better value proposition all around. UA no value to offer to the average flyer over WN on flights under 2.5 hrs (a large chunk of WN flights).

They tend to at least match their prices to the competition, and often competition has to bring down their prices to match WN (so all flyers benefit).

WN gave up on their strategy of using secondary airports a long time ago, so these midcon hubs are the next logical step. They have lower operating costs than UA/AA, so they can squeeze these two on some of their most profitable hub-captive quase-monopolized routes.

The timing is perfect - they've managed to come into a market at the bottom time and time again (2001-2002, 2008-9), hitting the competition where it hurts and gaining enough market share to make it worth their while.