Anderson and Duchovny both claimed that the timing of the film's release against The Dark Knight negatively affected its box-office return. Duchovny was quoted as saying, "I’d prefer if it was a huge hit, but there are mitigating circumstances. We happened to open on the worst day in the history of cinema - the second week of Batman. The only thing worse would be to open with Batman and nobody would’ve done that."
wikipedia.org on Star Trek Nemesis
Rick Berman - executive producer of the movie - has suggested that Nemesis's performance may have been negatively affected by "the competition of other films"
Not that I think that's the reason either movie performed badly - I think they performed badly because they were probably just bad movies. Can't comment on the X-Files (never saw it), but for Nemesis... I think the theory I buy the most is that it was a movie designed to appeal to non-trekkies but that non-trekkies weren't going to see it anyway, and that by trying to make it appeal to them, all-the-same, it wound up not even appealing to Star Trek's core audiance as much as it normally would have (which meant less repeat viewings).