Newcastle airport isn't easy to get to for everyone. People living south of the river don't find it that easy. They can also connect more locally via Durham and Darlo and to a lesser extent Morpeth. NCL airport is very easy to access for those north of the airport, unfortunately most people live south and east of the airport.
It offers one departure point for the people of the north east vs four points of departure for the train.
There are hourly direct rail links. No changing involved. Your car is also literally steps away from the train door. No faffing about with car park shuttles - which now take up to 15 minutes toe preach certain car parks at the airport. You can also drive it in just over three hours.
The market has spoken. If a 29 (sometimes 19 seat) seater can't make it work then I very much doubt anyone else can make it work. For most people in the real world cost is king.
You also can't put a price on finishing a meeting early in town and getting straight to the station and on the next train home. No hassle of getting to the airport. No waiting hours for the next flight home. With the train it's a 59minute wait worst case scenario. You can't put a price on that flexibility and convenience. Even better if you're in first where you can chill out in a lounge and once you're onboard you get some food (way better than the standard eastern airways cereal bar).
As always I hope to be proven wrong.