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[Warhammer 40k] Why there are not any Chaos Eldar, Orc or Tau forces?
gpt
The Eldar can be divided up into two factions – 

The “good” Eldar who eschew desires and walk a straight and narrow path to avoid any corruption. These are mostly your exodite and craftworld Eldar. Now granted these are still not very nice in appearance as they use creepy warp based farsight to take drastic  actions to prevent some unknown tragedy and they get pissed at everyone for not understanding. So “good” in this case means “not in bed with chaos” 

The “bad” Eldar or Dark Eldar barely survived the fall of their race and as such She Who Thirsts has her hooks in all of them. Now in order to avoid having chaos suck their souls out like a delicious Eldar Juice box they sacrifice the souls of others through… some pretty horrific torture sessions. Think of them as hyper-sadistic space vampires who instead of blood feast on your misery, suffering, and your soul. 

Now to the Imperium and probably most non Eldar – these two are basically the same. 

The Tau barely have a warp presence – as such Chaos tends to ignore them. Think of it this way – you are at Golden Corral – are you going to hit the buffet line? Or are you going to go scrounging for crumbs? Also the Tau don’t have a large enough presence in the Milky Way to really warrant the attention of Chaos.

Orks – follow Gork and Mork. Orks will fight daemons, men, Eldar, other Orks, Tau, Nids, and pretty much anything. The real magic of the Orks is – they won. They have no victory conditions, they have no weird species destiny, existential crisis, or desperate mission. They fight – and then they fight some more. If some bizarre reality warping happened and transposed chaos and the Imperium? The Orks wouldn’t care. There is no great struggle for them – just a good fight and a tall glass of fungus beer at the end of the day. Chaos can’t offer them anything – no one can. (Though there was that one Ork warband who wandered into the Eye of Terror – they are stuck on a daemon world where they fight and kill eachother every day and are resurrected the next day. The Orks think it’s pretty awesome.) 

TL;DR – “Bad” Eldar exist, Tau have small warp presence, and Orks can’t be tempted by Chaos.