More consumers are using kayak, bing/travel, and other flight aggregators. Airlines are competing on the price of tickets so they tack on extra charges at checkout. Southwest doesn't play in the aggregation game so they can factor the price of storing luggage into the price of the ticket. Southwest doesn't want to commoditize their product by making their flights available to aggregators and part of that is offering a differentiated consumer experience.

This isn't the point that the author makes but could explain Southwest's strategy with luggage....