Several factors were in play.
--Philadelphia and Pittsburgh as you know are both hubs for US Airways. The business communities in these cities both have the US FF
needle jabbed very deeply into their arms. Any Pennsylvanians who know differently please say, but I'd bet that the vast majority of air travel between PIT
is business-related. And the AirTran flights, if I remember right, were tried during 2000-2001, before 9-11, and businesses were less price-sensitive. So road warriors were in a position to demand being booked on US.
--US Airways is Pittsburgh's largest employer, and the airline's association with SW
PA goes all the way back to All-American Aviation. I lived near Pittsburgh for two years in the mid '90s, and got the strong impression that Pittsburgh has a lot of pride tied up in the US hub too. That's a psychological factor that can definitely affect travel patterns.
--US offered over 10 dailies, and AirTran offered four.
--Whether US just matched fares (legal) or predatorily dumped low fares (illegal) I don't know. No case was ever brought, that i know of.
The Pittsburgh Post-Gazette used to complain regularly about airfares when I lived there, but they might as well have been talking to a brick wall. Business pax wanted their FF
fix, and their employers could afford to support their habit. So a lot of people drove to Cleveland for WN
or didn't travel by air.
AirTran would probably still have a hard time making that city-pair work today, or as long as US exists. US would probably match prices as long as needed, and count on FF
loyalty to take up the slack. PA is the only Eastern state with two hubs by the same network carrier, and in PIT
is a fortress hub. Even when US shuts down the PIT
hub--which they will do, I think within a year--they will still have a strong RJ
presence, and a hub in PHL