Probably because everything sold out is my guess? People hopped on these sales a few days ago. The seats (at the cheaper fares) are selling out. I hope you all don't think US is selling all the seats at 98 dollars a pop. There are a small amount of seats that are sold. Once they are gone they are gone. That is why I bought all my tickets last week.

DCA to FLL 78 bucks roundtrip ^ Way lower than any other airline from DCA to the Miami area. (last week of course)