More specifically, Troll Bridge is a fan film - on which not a single person is getting paid*, so work on it is purely a passion project on behalf of hundreds of crew, cast, and post production workers.
The productions being run by Narrativia are progressing, but as with all TV/film projects, they also need financial backing, which is I think mostly where things have stalled since the original announcement. Without a TV station backing such projects never really get off the ground. Quite often you hear of book/comic adaptations being "made into a movie/tv show" and then nothing happens for years, and it's usually because the people with the money aren't prepared to fund it *right now*.
Take for example the American Gods TV show. A few years ago it was announced that it'd be an HBO series (as a follow up to their successful shows True Blood and Game of Thrones) but earlier this year it turns out that deal ended up falling through, but the Starz network decided to fund it instead. The same production company is probably making the show, but it's just partnered with a different TV network (for the financial backing)
*except for the helicopter pilot.