Board Thread:FAQ Board/@comment-6697950-20150424092122/@comment-27384850-20160714045621

Another option here is that they plan to release the arc as a single, massive season like what was done with Fullmetal Alchemist: Brotherhood. If that's the case then it's only natural that they have to wait for the conclusion of the arc before animating it. Perhaps they've already got a lot of work done on it, but they can't finish it yet because there is no conclusion yet.