So, I am wondering, not as much from an RPG standpoint (because Human is mostly what we play, so I get that), but a fluff one, why is it that the Ruinous Powers of Chaos can only hold sway over humans? This is an issue I've often had with the 40K fluff. Yeah, Chaos has daemons aplenty, and numerous human Cultists at their service, and even a large percentage of the Space Marines said "screw the Emperor, and all we were indoctrinated to believe, lets serve Chaos!", but Humanity is but one race of many in the universe, and the forces of the 4 Gods seem to pretend that the rest don't even exist.
If I was Khorne, I'd be looking at humans, then looking at orks, and I would say "get me some of them greenskins". They fight, they are built to fight, they love to fight, and they can shrug off more damage than most other creatures I can name, plus they breed like mad, like to collect skulls and trophies, and on, and on, and on. Tyranids are the greatest threat the galaxy has ever faced, potentially; a massive force, completely bent on consuming everything, able to breed quick, absorb punishment, and resilient to many means one might use to eradicate "pests". They can't be intimidated, frightened, or given to diplomacy. If I were Nurgle, I'd see them as quite the wave of entropy, a force that could tear things down, and leave nothing behind.
So, why do the Chaos Gods seem to just 'settle" for Humans? For Orks, I don't like the idea that they already have two gods. If Slaanesh could wipe out the entire pantheon of Eldar deities, and he was newborn, and is the weakest of four gods, then one would think that the Wargod Khorne could topple two stupid Ork gods, who may or may not even exist, if not impersonate one of them, and trick the Orks to serve. If powerful, refined, well trained Space Marines can fall, the Orks should have little hope. The Nids have a bit more resilience, with their Hive Mind, and Shadow in the Warp, but I could see Nurgle eroding that defense, over time, and getting in. There's little bits of Genestealer Cults falling to Chaos, but I don't even know if that's still canon, and would think Chaos could do more.
Tau are rather resistant to Chaos, too, and barely sign in the warp at all. The Eldar have been touched by Slaanesh, but none would consort with him now, minus the foolish Dark Eldar, and Necrons have no souls, and so no value to Chaos.
Thus, what GOOD reason is there why Chaos doesn't dig its claws into some other, maybe better choices of servants? Why does practically every other race have a lame, built-in defense against these enemies, leaving Humans as the only viable prey for the predations of the warp? Okay, thanks much.