Animated sequels are tricky
An animated movie sequel can always be hard to pull off, and the sad fact is that many have diminishing returns (see the Shrek movies). Frozen II was fun but not like the original and even Pixar has trouble pulling it off. Moana seemed to be a classic "one and done" tale, and trying to continue it is tough.
Given this was never meant as a movie in the first place, it makes it worse as the story has been reworked, and it runs the risk of "replaying the hits" in the story and maybe even the dreaded "retcon" of some characters. This has the concern Moana 2 may only tarnish some of the goodwill of the first film.
The live-action version can be a distraction
It was surprising to many to hear Moana would get a live-action adaptation given how the movie isn’t even a decade old. There is some speculation that it was really a cover for this sequel, yet Disney appears to be going ahead with it. So it’s a live-action version of a movie that now has an animated sequel.
How many are going to come out for either? Live-action musicals are having a nice boost lately in movie theaters with Wonka and Mean Girls, yet Disney’s live-action record has been so-so, to say the least. To push two Moana projects at once seems way too much for the company.