User blog comment:LilyheartsLightning/Citadel and Reckoning DLC announced/@comment-1786741-20130221185148/@comment-5521671-20130221193324

Not every story can be told in the original game.

If we use your presupposition things like Overlord, Arrival and LotSB should also have been in the original ME2. After all they fill holes left open in the original story.