Interestingly, Tremulous died at around the same time as True Combat: Elite. I think it was just overall the time for it to go.
In 2011, we had games such as Skyrim, Modern Warfare 3, Battlefield 3, Dark Souls, Crysis 2, Killzone 3 being released. It's difficult to see how the likes of Tremulous or True Combat: Elite could have continued to attract players against those kind of games.
There was also kind of a lack for both games in community leaders and managers. Small gaming communities live or die on the mercy of just a few players. There are key characters who lead the whole of the community and lead the development of these games. When these characters leave, the communities tend to crumble if a new leader doesn't step in. This new leader never really properly stepped up Tremulous. Those kind of people are 1 in 1000s.
Overall open source games and open source game development are in an all time low. The gap of budgets between commercial games and truly free games are just getting bigger and bigger. I do not foresee any kind of a resurrection for open source gaming in the near future.
