This is all just conjecture, but my developing theory about why Disney is releasing the originally-planned 'Moana sequel series' as a 'theater-bound Moana sequel' is that Disney is scrambling to regain the audience goodwill it lost after releasing Wish (alongside several ongoing boycotts against the company.) It would explain why there's not even a year between the announcement and the release date.
Disney+ is also struggling to justify its own existence to consumers, just like most current streaming services right now. So streaming-exclusivity is currently where new Disney content goes to shine briefly before dying with a whimper, and some higher-up probably thought a Moana continuation deserved a better shot than that. We might be seeing more of these budget-shows-turned-movies in the upcoming years due to similar decisions. This would parallel the Disney era of direct-to-dvd 'movies' that were actually compilations of TV shows that were cancelled mid-development. If this does occur, we may officially be in another 'dark age' where Disney is actually struggling to maintain itself (by Disney's own corporate standards, at least.)