I feel like this phenomenon is due to three things.

1. A refusal to grow up. Not that it's inherently childish to enjoy lighthearted stories, but there's definitely a draw to all the Peter Pans out there that don't want to engage with something that'll challenge their worldview. Having a healthy disconnect and ability to recognize the difference between fantasy and reality is kind of a requirement to engage with darker stories, which acts as sort of auto-gatekeeping.

2. Going to where the children are. Or at least, where they think they'll be anyway. For all the pathological ideologies out there that reproduce via viral infection, you go in and try to hook the kids to your way of thinking early. This naturally leads to the fandom acting childish, due to literal children and people using the excuse to act like literal children. Adult-aimed stories will, ideally, be followed by adults who already have a level of maturity that make discourse more pleasant. Granted, it depends on whether it's "Adult" or "Mature."

3. Safe spaces breed discontent. When positivity is mandatory, stepping out of line brings down the hammer for disrupting the peace. Thus the purity spiral forms and everyone starts hating each other for not living up to the impossible standards imposed by the community. When people come in expecting there to be disagreement and dissent, discussion can happen without shutting down risktakers and independent thought.

This is definitely stuff to think about when making media. Try to draw in a healthy audience that naturally rejects the false positivity mindset. Even if you're making something fluffy, give it some meat to chew on underneath that'll stick in the craw of the cotton candy chasers.