I imagine the demand is not sufficient to support higher fares, and low fares are won by WN. actually, i'm sure high fares are won by WN as well since they have many more flights and casual travelers tend to prefer them v UA. i know my aunt just flew out of there and paid $500 one way to go to MDW. i would never pay that much for a one way flight to chicago on any airline from any airport, but WN seems to generate loyalty such that folks will spend more to fly them.