These are nt combineable because the airline wants to compete with the LCCs only on specific routes, like DCA-MCI but not make those fares available in other circumstances. For example, if you were looking to fly EWR-MCI and the DCA-MCI fare you found were combineable the EWR-DCA-MCI routing might end up costing less because of the LCC-match fare on the DCA-MCI leg. UA doesn't want to give up revenue like that.