Cause the Manga and Movie clearly showed it was Momoshiki that turned him into one, just like we saw it was him who turned Hachibi's chakra (or was it actually the Hachibi himself?) Into a chakra pill. Cause there's nothing saying that, and he did say afterwards to Naruto that he's next. Cause it makes the most sense, if you think otherwise then that's your opinion and this is mine. That's why I said it's my opinion and it makes the most sense.
Didn't act like anything don't get upset now, I can state my opinion if I want and this makes the most sense to me. If you disagree that's on you I don't really give shit.
It's not that I don' t agree with your explanations, I just don't like them, they make the manga look like horsecrap, that' s why I thought to other hypothesis than the 2 you mentionned:
-Momo being dumb to the point he forgets to use his most powerfull technique although he had many chances to save his life doing it.
-Kishimoto being dumb enough to forget Momo' s ability.
Those aren' t satisfying hypothesis at all, they mean that the manga is bullshit.
If you are satisfied reading bullshit, it' s your problem.