Just to be clear, Southwest is not discontinuing service to Manchester (MHT), Boston (BOS) or Providence (PVD). All three airports will still have SWA service. They are dropping routes to Philadelphia from all three cities.
The problem was that US Airways also flies all three routes. US matched SWA's fares and bettered the frequency for the routes. US also uses the three routes as feeders to their PHL hub for both domestic and international service. Business travelers generally do not care for the SWA frequent flier program with its limited routes, no first class upgrades and no alliance. The bottom line was that the fares on all three routes dropped, but travelers by and large stayed with US.
With the merger between AirTran and SWA, SWA took a good look at all their operations and decided to pull off the PHL routes and generally reduce their presence at PHL. When SWA came into PHL in 2004, US was a financial basket case. It was widely forecast that SWA would drive the final nail into US's coffin by running US out of it's most lucrative hub operation. It did not happen, and now eight years later, is US Airways is alive and well, and SWA is cutting back at PHL. What goes around, comes around.