One of the great problems with the 40k franchise is that, really, they can't show Chaos. Not as it should be. Oh, sure there are lines from the Ultramarines books (particularly Dead Sky, Black Sun) that made me sleep with the lights on for a night, but let's be realistic here: that's only because it's a book and nobody's rating it. You can't do Chaos properly in a game. It'll **** up your rating something awful. You may not even be able to sell it.
Games Workshop has also demonstrated remarkable prudery when it comes to Chaos. Oh, sure, there are Nurgle descriptions to put you off eating for a few hours, and Khorne can get pretty lovingly detailed messy on occasion too. But Tzeentch? Nah. They don't have anything good for that. Probably it's just hard to describe, but by this point it has to be at least partially delibrate since we've got plenty of Lovecraftian Horror writers out there to show how it's supposed to be done. And their description of Slaaneshi stuff is a complete joke. (Which is sad to all the teenagers, sad to me since I could rag on them for outdated sexual mores, and sad from a storytelling standpoint because damn there is a lot of stuff you could do with that whole sensation-enslaved thing which would make even me get really damn angry and want to hunt down and kill Daemonettes as perverted evil freaks...rather than make Slaanesh basically toothless.)
So a Chaos campaign was impossible. They have to limit your viewpoint, to keep the game something they can get rated and therefore sell.