The reason for this is simply because there's not a whole lot id Tech 4 has to offer that other open-source engines don't already have. The previous id engines are better understood by the community, and many of them have already been upgraded to match or exceed id Tech 4 in terms of graphics and performance. The community-extended engines also generally have better cross-platform support.
Even if you ignore the other engines having a head start, id Tech 4 has its own inherent disadvantages as well. For starters, it's huge: at more than a half million lines of code, it's 2x as big as ioQuake3, 3x as big as Qfusion, 3.5x as big as DarkPlaces, 4.3x as big as CRX, and 4.9x as big as Quake2World.
If you wanted to start a brand new game from scratch using an id Tech engine, and you didn't want to use any of the community engines, id Tech 4 would probably be the best choice. But existing games don't really have any reason to switch, and for new games, the id Tech engines are going out of fashion. Unity, Unreal, and Source have largely taken over what used to be a largely Quake-dominated modding scene.
For that reason, I predict that the only open-source games we'll see using id Tech 4 are Doom 3, Quake 4, PREY, ET:QW, and so on (depending on which ones get open-sourced.) Open-sourcing id Tech 4 was a good way for id Software to keep existing games alive for decades to come, but I don't think we'll see any new long-lasting open-source games with that engine.


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