I would also add that this film short needs much tighter editing (my sense is that this could be cut down to 6 minutes), and a lot of the footage is dark, muddy, because there wasn't enough lighting on set.
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.