Good grief, there is no mistique here, airlines are commercial enterprises, they maximise revenue and need to make a sustainable return. They are accountable to shareholders. Simples. Whether United made a profit or not, the reality is that the profit / yield was not to their liking, and they exited. The market is to leisure focused and extremely seasonal. That's it in a nutshell.
Norwegian while it can stimulate demand, it only is point to point. So in a sense not exactly what the airport needs.
Virgin have been acutely aware of the seasonal factor and as a result been building the service to be in a limited basis and now it has been extended. Hopefully they can earn a sustainable return across all months.