The demise of the Pittsburgh hub is a sad, sad, story - almost as sad as the demise of cities like Cleveland.
Some points -
The Pittsburgh market wasn't big enough for the folks doing origin/destination type travel, which is important for a hub (folks flying with a PIT - ... Itenerary as opposed to connecting).
Folks on the East Coast, even though they heard Pittsburgh wasn't that full of air traffic delays, weren't really interested in flying 1 hr west to then fly somewhere else, especially all of the Europe Destinations (London, Paris, Frankfurt)
US Airways eventually saw big East Coast Cities like PHL was where the $ was and moved more flights there.
I think too that they just had way too many flights to smaller airports in the western pa/northeast Ohio region - Youngstown-Pittsburgh and Latrobe-Pittsburgh come to mind. Heck, they even had Reading PA - Pittsburgh etc...
On Airliners.net there is a huge Pittsburgh crowd who was debated/written at great lengths about the demise of PIT. Sure, there have been airports that have shrunk in size, or medium airports with smaller hubs that went away (think America West/Columbus) but for such a new airport to be so huge, then shrink to so little size in only 10-12 years after opening, wow...
The silver lining for PIT folks is that the loss of the hub brought in WN and grew Airtran (I think they were around during the hub days - in fact - they briefly did PIT-PHL, bringing the fares down from $1,000 roundtrip) so folks from PIT do have affordable flying - just less destinations.