Because Kishi isn't Chief Director of the anime. He stated himself that the Last is tied in with the manga.
It's stated as the final chapter in Naruto's story. It's canon.
And you think Kishimoto can animate?
Kishimoto is not directing the movie.. hes providing a storyline for it.. Just like he is the one who provides the storyline for the anime (yes each episode does have a writer, however that is to divide the episode and add anything into it. )
Just because the original writer is making something off of his work .. doesn't mean its automatically "canon". Because people who work their asses of for the anime, were given permission by Kishimoto himself.
I mean i don't really care whether its canon or non-canon. Its just the whole idea about "if the original writer writes it, its canon".. that doesn't go well with me, when the original writer is giving off copyrights to his stuff to be edited and added stuff into.