This movie makes no sense to me. Why are there so many random Sinnoh references? The new companions are both from there, their mascots are both overexposed Gen 4 Pokémon, and the new girl's main plot point is even being the daughter of a certain overrated blonde Champion from there.
(Which, as an aside, makes her not only a punch in the gut to Misty and Dawn, but also Lance. The girl literally only existed to shoehorn in a reference to a certain already overexposed Champion from an unrelated region that could've instead gone to the actual Champion from Kanto and Johto, who is a very ignored, neglected character.)
Why so much Sinnoh? It doesn't appeal to OS era nostalgia, nor does it promote the current generation. And, remakes are still a few years away and probably won't happen any earlier than 2020. It's just an old, irrelevant gen that nobody but diehards really care about right now.
If they wanted to "celebrate more than just Kanto and Johto," then why not just set the anniversary movie in Alola and use the SM cast? And, why give all of this attention to Sinnoh, specifically, but not Hoenn, Unova, and Kalos?