Its quite simple, many airlines are doing the same thing. These days US airlines want frequency rather than capacity. This is why you are seeing some 757s being down-graded to 737s, so they can add a couple more flights.
These days passengers demand flexibility. They want to be able to fly to there destination when THEY want to. In-order to do that, airlines have to down-grade to smaller planes, and add more flights. I know that Delta has plans in adding more flights to Denver (DEN) from Atlanta (ATL), and yes I noticed, that I'm seeing less 757s, and more 737-800s now.
But don't worry, there will ALWAYS be a place for the 757.
The 757 is one of the most valuable aircraft for US airlines.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran