system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Why aren't there more Chaos aligned factions? Surely some factions or races other than the Space Marines have fallen to the powers of the Warp.
gpt
Most other major races can't or don't often fall to chaos.

Eldar-terrified of slanesh and place their souls into soul stones to keep them safe.

Dark eldar-not technically chaos aligned but sort of close. Torture others to "top off" their soul while Slanesh constantly drinks it.

Orks- Gork and Mork are the only gods they need (though these two might be minor chaos gods its not really clear) also they would need to stop fighting long enough to listen to the gods or their representatives. 

Tau- their souls are too small to be worth the effort for the chaos gods.

Necrons- Ctan beat the chaos gods too it. Also Necrons HATE all things warp related.

Tyranids- debatable if they have souls but if they do they don't have the individuality to swear themselves to the gods.

That said there are probably rare examples of some of these races and minor xenos races in chaos however due to the human centric nature of the lore they are rarely if ever mentioned.