Could be a lot of things, but my guess is that they are suffering from the woke backlash that seems to be the focus these days on nearly anything coming from Disney. I think people are fed up with the politics of the films being more important than the story. As you mention, the advertising was poor and was more concerned with the fact that there was a non binary character than it was with the story and people don’t care anymore. They just want to leave the politics behind and see a good film.

This article discusses it a little

https://www.them.us/story/elemental-disney-pixar-nonbinary-character-lake-ripple#:~:text=Hauser shared their casting on,if you wanna see them.”