In S2E1 of TLA, Roku tells Aang that the Avatar state is the combined power of his past lives and if Aang dies in the Avatar state, then the cycle ends. So with the revelation in LOK that the Avatar State comes from Raava, why is that?

 My personal theory is that Raava would not be given the chance to find a new Avatar. Similar to when she is killed in season 2 of LOK.

 I also think Roku's lack of knowledge or mention of Raava could be explained by the thousands of avatars that have gained knowledge to share. Perhaps at a certain point Raava was no longer needed for Avatars until Harmonic Convergence when Korra discovers the spirit again.

Come to think of it, when Korra loses her connection to the past Avatars she begins to speak to Raava as her guide, like Roku to Aang. Perhaps it is Raava all along simply communicating through familiar faces and avatars. In Korra's next life Raava and Korra will guide the new Avatar together in the form of Korra.