They haven't changed the mileage requirement, but its more like the tickets priced at those miles sold out. Wouldn't be much different than if you saw the flight for $200 on Friday and three days later its selling for $400.

Helpful hint for the future -- Jump the moment you find the good deal, especially if you have specific dates you need to travel. Don't expect them to be there if you wait, airline inventory is constantly flucuating.