Ultimately, this isn't about what show "deserved" a movie, but about simple practicality. DS9 was a great show, no question, and arguably much better and more involving than either VOYAGER or ENTERPRISE, but movie-making is not a meritocracy. It's about popularity and it was obvious early on that DS9 was not going to be as popular with general audiences as TOS and TNG were, so a DS9 movie was never going to happen. (Ditto for VOYAGER and ENTERPRISE.)
That's no slam on DS9, which deserves all the accolades it gets, but just cold-blooded pragmatism. I'd love to see an "Assignment:Earth" movie, too, but I'm not holding my breath.