The X-Files: I Want To Believe was held up for several years because of lawsuits from David Duchovny and Chris Carter against Twentieth-Century Fox over syndication revenues. (Fox sold the rerun rights to FX, part of the greater News Corporation conglomerate, instead of letting other networks compete for the chance to rerun the show; once that was resolved, they didn't compensate profit participants including Carter according to the deals that were made). If those lawsuits hadn't occured, I bet a movie would have happened earlier, and been made for a bigger budget. As it happens, it had been several years since the series ended when they made the second film, and Fox wasn't confident that the fanbase existed to support a big-budget movie. Either they were proven right or the low-budget, standalone movie they wanted became a self-fulfilling prophecy, but the movie failed hard at the box office. That's going to be quite a hurdle for a prospective third film to overcome, although stranger things have happened.