It is what they want. It is part of an idiotic plan which they have been trying to foist since 6th Edition. The problem, which they refuse to accept, is... 1) Nobody has to buy anything from them to add these allied detachments. The glut of models on the market (legit and not so much) make it unecessary to go to them. 2) People are not playing larger games. They are embracing smaller games because of clunky rules and the impossibility to keep up with all the rules sources. 3) They have managed to bury nearly all good will which makes people even less inclined to favor them and find it easier to justify less than savory sources.
Since the end of 5th Edition, wherein the average game size in my area was 2500pts, games have shrunk to between 1500-1850pts for almost every single organized game. Most pick up games (what few there are) mirror the event sizes as most people want to practice and try out their lists for the tournaments. Moreover, Tournament Organizers have (because of bad, vague rules writing and spam rules) limited sources. These two pragmatic truths deter massive purchases or the need to expand into multiple armies for the average player. There will always be fanatics, but most people play ONE army with at most a touch of some other detachment. That detachment doesn't require Games Workshop.
For example, I can easily "tart up" some of my Terminators to be fallen GK. Hell, that is a much better use for CSM Terminators (turning them into GK gone bad) than their natural state. I've already set aside the models, raided my bits for the appropriate guns, and presto chango I've just added more Warp Charges to my army and access to Void Grenades.