Frontier was the last major airline to use New Castle. They started up there around the same time they started in Trenton, and shortly thereafter in Philly.
They've done very well in Trenton. They are thriving in Philly. They pulled out of New Castle.
Seems to be there's a reason why airlines can't survive in New Castle, yet they can survive in other smaller and larger airports.
As for the airport itself, maybe they are over-exaggerating themselves. I've mentioned this before: Take a look at their webpage:
http://www.newcastleairportilg.com/ . It states they are only a 25 minute drive from Center City, Philadelphia. The shortest distance to City Hall, Philadelphia is 34 miles (US 13, I-495, I-95, PA 611). In order to make that distance, one would have to average 81 mph, and that includes time spent on US 13 and Broad Street in Philly. In reality, the approximate time for that distance is nearly 50 minutes. If you try to stay on the highway as much as possible (US 13, I-495, I-95, I-676), then it's a 38 mile trip, needing to average 91 mph. That trip should normally be about 45 minutes.
So, where the airport website gets 25 minutes...who knows. But it's probably a selling point that travelers who have options have found untrue, and have decided to take their business elsewhere.