I believe the "key" to United's success is because of their hubs. I mean look, IAD, ORD, DEN, SFO, and LAX. These are big industrial and economical cities; as well as being gateways to the other corners of the Earth. Sure, other airlines have some hubs there too, that's why they aren't like, the 8th largest airline. United and American are the two largest and successful airlines. Look at their hubs! They also offer more flights than the others (airlines).
I have also noticed that US Air has a larger route network in the Northeast than the other's. They have hubs in PIT and PHL, two big industrial cities. US Air also has more direct flights in the East; that's what I have noticed anyway.
I believe that US Air really dominates the Eastern US, as far as flights are concerned. United dominates the West. Look at their hubs!
The key to starting a successful airline is hub choice! Don't go around saying that successful airlines are crooked and want to squash any competition. The successful airlines just run a good business!