The rest, such as Frontier, JetBlue, Southwest, Allegiant - are all discount and/or regional airlines.
For example, allegiant flies to most cities secondary airports that are 1-2 hours drive from the city.
EDIT: It's really 4 equally large ones by passenger miles:
* 1 Delta Air Lines 17.8%
* 2 American Airlines 17.5%
* 3 Southwest Airlines 17.3%
* 4 United Airlines 16.0%
The rest are sub 10%