Limited capacity airports that are in highly desirable locations (bascially DCA and LGA). The idea is that if you are in BOS, the convenience of DCA over IAD/BWI means far more than if you are in SFO. The SFO traveler is rarely traveling for one meeting, lunch, dinner, a day trip. The BOS traveler may go to the office in the morning, fly to DCA for a 2 p.m. meeting and be at home by dinner. The location of DCA matters far more to them than the SFO traveler who comes for a week.