Southwest has increased service minimally at PIT since it's start. Don't know the reason. WN has stated that if gates were available, or when they become avaiiable, they want to increase the # of flights out of PHL to over 100/day. For the period 7/2005 through 6/2006, the number of Southwest enplaned from PIT was .979M and from PHL 3.1M.
My .2 on PHL. If you read the many, many posts on the many airline forums, the comments/complaints are really no different than most every other Hub. Many FFlyers hate their Hub for the delays, congestion, weather, etc., but if you ask if they would rather have it closed and connect through another city, rather than have worldwide access at their doorstep, they fall back into the good soldier posture. I'm not really certain how many people realize who would pay for a new airport. The answer is the passenger and the airlines. That is why PIT landing fees are so high - even today, because there is still a huge debt. The other point (as someone made earlier) was that PHL was burned by US during bankruptcy and then there is the history of how PIT was so unceremoniously de-hubbed. USAirways is really still an unknown to the city as far as viability is concerned, so why should they invest $1+B on the basis of such short term experience? Also, if you examine the current state of the industry, what would happen if for example US and UA merged. It's very possible the PHL activity would be downgraded drastically in favor of IAD and so forth with other potential US mergers. Even though Parker and the US analysts have stated many times that PHL is the "crown jewel" /"premier" hub for the airline, I'm not certain they have shown much initiative to improve the facilities therein. My opinion is that US wants to be an LCC as far as spending is concerned, but also wants to be a global player in the industry. It really needs to decide which one it wants to be and go from there.