Domestic travel has become a commodity with a very elastic demand curve. Furthermore the way most people buy tickets these days, internet resellers, hinders individual companies ability to raise prices. Go to CheapTickets/Hotwire/Expedia and search for a flight. They rank order flights by price. One dollar difference could put you on page two. Unless all the players agree to raise the price you lose market share. Add to that the LCC undercutters and you see the problem. That is why the majors are back dooring people with added bag charges, fuel surcharges, etc.
Just my HO and junior economic analysis.