Galdred
Studio Draconis
It is because fantasy space Marines just don't really work.The only saving grace of AoS is that the MUHREEN faction doesn't actually take center stage like it does in 40k, but I guess the guys behind this video game don't get it.
And they kind of existed before. They were named Chaos Warriors.
Just look at the beginning of Warhammer Online, when everyone was rolling a Chaos Chosen.
That is also why Chaos as launch DLC for Warhammer Total War made fans angry (but I suppose it would have been the same with all factions, except maybe Vampire Counts).