A typical TNG feature film wouldn't be able to cover the amount of special-effects budget needed to make a Q film, and the character was more interesting from a performance/writing point-of-view in the past TNG episodes.
As it has been mentioned before, John de Lancie as Q isn't a big enough draw for the mainstream ticket buying audiences in America and around the world.
With Generations, the big selling point was William Shatner's teaming with Patrick Stewart and his final swan song; First Contact had the Borg; Insurrection (as lackluster as it was) featured F. Murray Abraham (But, did his casting really make a difference to this film???); Nemesis cast Tom Hardy as Shinzon (They knew his box-office potential was there when they cast him back then.).