Yields. DFW and ORD have a bigger, higher-yielding, more business-oriented O&D base. They want the low-yield traffic to go throw St. Louis while Chicago and Dallas get the higher-yielding traffic. While St. Louis yields do suffer, it is well made up for by the yields at DFW and ORD. For example, why sell the cheapest Y fare on, say, ABQ-DFW-MIA, when it could go to a last minute busienss traveler doing ABQ-DFW and another one doing DFW-MIA? Instead, just route through STL which is more unlikely to have that last minute business traveler.