I think its due to US Airways virtual monopoly at its Pittsburgh hub:
Look at all the cities monopolized by just one carrier:
Dallas, St. Louis, Pittsburgh and Charlotte, Minneapolis.
You will find pretty much the same thing:
No competition on most routings (especially long hauls) and truly outrageous fares!
Dallas is a little better because Delta is at least a solid player there, though not nearly what they used to be.
One interesting note about Pittsburgh: United and TWA were strong players there in the 60's and 70's and had most of the primo routes to places like Denver, San Francisco and LA. Allegheny (the original airline that became US Airways) was relegated to short hauls east of the Mississippi. But after deregulation, both airlines quickly abandoned Pittsburgh, even though they could have easily competed against "US Air" as it was called in the early 80's. The reason: Supposedly because those two airlines were more interested in monopolizing hubs of their own, and service to Pittsburgh has never been the same since.