If United's offer of $800 wasn't good enough to entice any of the hundred(s) of checked-in, boarded, and seated passengers on the plane, and the "randomly" chosen few had to accept their fate, then United could well offer only $50 next time and the "lucky" ones chosen had to accept their fate.
It was clear the offer (probably vouchers with bundles of restrictions that were difficult to redeem) wasn't good enough to NOT even one single passenger already on board.