I still don't understand what people don't grasp here:
If you run mainline to LGA, you need to serve big markets inside the perimter, which means you are competing with American, Delta, JetBlue and Continental in the NYC market. Cincinatti, Cleveland, Detroit, Baltimore, Columbus, Nashville, Raleigh, Pittsburgh, Indianapolis, etc etc are all served by multiple carriers, in most cases at least 3 of the 4 mentioned. It's insanely cheap to fly to NYC, even next day, from any major market.
The only cities that have moderately high fares are smaller markets, which don't generate the passengers to justify mainline service. Due to higher operating costs, even with the moderately higher fares, most of these flights aren't profitable. The fares still aren't high enough due to at least 1 other competitior on almost every route.
DCA on the other hand has a much higher competitive advantage, given the premium to fly their as opposed to IAD or BWI is greater than LGA is to EWR or JFK. In fact, I prefer flying to EWR and taking the train, as opposed to taking a cab from LGA because I don't need to worry about traffic.
There are far fewer compeitiors in the market offering competition to major markets. There is usually not more than 1 or 2 competitors to major markets, meaning fares are higher. Combine that with the extreme preference for DCA, and fares are tremendously higher to DCA than to LGA.
Honestly, it seems that people on here think its ok to treat the New York market as some sort of loss-leader, and in some respects I understand that, but in an industry with as volatile earnings as an airline, I think its prudent to focus on financials as opposed to perception.
Let'd face it, US Airways is 4th in New York market share. Remaining 4th (or falling to 5th) isn't really going to hurt the brand, as much as dominating DC will help their brand.
