Actually, if memory serves correctly, Southwest's pull-out from
DEN happened for a couple of reasons. As others have stated, delays at Stapleton (and the impact on the rest of the company's operation) were one reason, but
WN was also competing with
three established hub-and-spoke carriers at
DEN in the mid-1980's --
UA,
CO, and Frontier (I). And in that competition,
DEN-area passengers chose the other airlines when they matched Southwest's fares, making
DEN unprofitable for
WN. Now, remember that this happened almost 20 years ago when Southwest was a far smaller, less financially robust airline with a far weaker network, and they didn't just pull out after a few months then, either. Pinning your hopes on
WN pulling out of
PHL in the short term is unrealistic; after all, they stayed at delay-plagued
SFO for nearly two decades. The issue of gates at the new
DEN had nothing to do with it, since the new airport was little more than a long-range planning concept (if that) in 1986.
WN has had little interest in serving the new
DEN due to (1) excessively high rents/fees and (2) the presence of a large network hub coupled with a good-sized pseudo-LCC hub (though Frontier has moved closer to being a true LCC in the last two years). The same is true for
ATL aside from the high fees.
Why is
PHL a more attractive opportunity for
WN? Well, for one,
PHL is clearly far more underserved than
DEN, with only about 75% of
DEN's O&D traffic in spite of a metro area that was nearly 150% larger as of 2000. Average fares at
DEN are considerably lower than those at
PHL (which partially explains the lower O&D at
PHL).
DEN has two hubs as mentioned above, while
PHL has only one network hub and a relatively limited (~ 25 daily departures) LCC presence.
PHL offers a nearly unique opportunity (along with
BWI and possibly
PIT) for
WN to expand its service to smaller cities in the East;
WN already serves most of the cities out West which can support 10 or more daily 737 round-trips (aside from
COS,
DEN,
SFO, and maybe
FAT).
I suppose that
PIT could be the new city if US does go ahead and reduce service there as they've threatened. Most of US's gates in Concourses A & B at
PIT are now on month-to-month leases (they only signed long-term leases for ten after rejecting the old leases), so clearly the Allegheny County Airport Authority has at least 40 gates at
PIT which could be made available to
WN. Per-passenger costs at
PIT are currently comparable to or slightly higher than those at
PHL; if the county does manage to reduce airport costs, that would make
PIT even more attractive to
WN in light of a hypothetical reduction in service by US.