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No World Eaters, or Thousand Sons.
Instead we have legions that are all just evil space marines in terms of tactics. Kinda boggles the mind. It's like Chaos but without all the fun.
The Iron Warriors are just boring in the context of playing as one dude. They have a gun. That's it.
The Word Bearers could be fun, but with no daemons to speak of they're just dudes with guns too.
Now those of us with grey matter know that the real reason for this is that it would be hard to balance a legion like the World Eaters who literally, if they get close, turn 5* their number of normal marines into goo.
Even more so with the Thousand Sons who just have ranged capability that makes any man portable gun looks like a super soaker.
Even then there are weaknesses to balance things with. Thousand Sons are just a marine with no melee weapon when cornered, and are few in number (limit on respawns maybe?) and World Eaters have no ranged capacity.

Wouldn't this be better suited for the Eternal Crusade forums?

Edit: The chances of getting special snowflake Chaos Marines in EC are slim. You said it yourself, World Eaters and Thousand Sons would erase regular marines in their respective areas of specialty (melee and shooting respectively). Even if they were represented in some way, the overall fanbase would continue to complain that they weren't done correctly - truthfully, it'll be better if they have them as NPC/Bosses/Elites you have to fight. Same goes for Vanilla Marine Veterans, Terminators, and other scary stuff that's WAY too good for a player to have.