Both those shows have significant followings on Cartoon Network and Neither is really making big bucks in DVD sales.
Further, being a prime time flop doesn't really have anything to do with quality. Witness Firefly. AND, a serialzed Television show is vastly different from a feature length movie.
Actually, Futurama had excellent dvd sales. Enough to justifiy direct to dvd movies. Fox doesn't make movies for fans. They make them for money.

I remember Firefly DVD sales were so great, that Amazon was often sold out of them. And I never would have thought to watch the show on tv (like most).