Veronica Mars is probably an exception here, because Kristen Bell and Rob Thomas both wanted to do a continuation. Secondly, this project has a fanbase from a series that ran for 3 seasons. Third, they weren't asking for much: $2 million is the average these days to fund a low-budget feature film. I doubt that an X-Files 3 or Judge Dredd 2 could be made for this low amount, and I doubt there is a strong enough fanbase to fund at least $20 million (at least at this point) for such a movie.
Additionally, trying to do this with most other TV series would be more difficult, I would think, because either too much time has passed since the cancellation of the series, or the main creatives involved -- actors, producers, writers -- aren't interested anymore, and have moved on with their careers/lives. Joss Whedon shot down the possibility of continuing Firefly through fan-funding because of this reason.