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BoeingBoy (retired pilot) can probably provide the best answer, but my guess is that the plane was full of passengers, bags and cargo and thus couldn't take on enough fuel to make it across the country. And they knew it from the get-go. Airplane capacity generally requires some tradeoffs. AFAIK, there are very few planes where you can completely fill the cabin, cube out the cargo bays with heavy bags and freight and still fill the fuel tanks to the top. And that's because there's a maximum takeoff weight that must not be exceeded, period. Yet full cabin, bags and fuel tanks would exceed that limit.

Another big factor this time of year is that jetstream winds tend to pick up velocity in the winter months, making it more difficult for A321s and A320s to make nonstop westbound transcons without stopping for fuel. Happens to jetBlue quite a bit in January-March (nothing but A320s and lotsa transcons).

Complaining would be a complete waste of time. It's not an uncommon occurence in the winter if you're on a westbound transcon on an A321 or A320. Miles will generally be awarded for the nonstop distance.