Why? We don't even know how much of the next expansion is created yet. We need an Alpha/Beta to first know how much is done and how much is left to do.
They doubled their dev team for Legion. Legion released late after WoD because they had to train all those new devs. Now those are trained and in full dev mode. They can pump out content faster than before (proof is the 77 days patch release, every streamers/youtubers who cover Legion all all saying they have no time to breath between ptr and live content cover videos.)