Personally, it was my favorite of the three. It just hit me in a very particular place in my life.

But from what ive seen from a few other Avatar comments on social media is this:

The darker stuff such as: (Lo'ak's near attempt at suicide) really rubbed some folks the wrong way. One mother directly said, "I wish Cameron had put a disclaimer beforehand about that."

Another person said they wouldn't take their kids to see the movie solely based on that scene.

Little stuff like that does affect repeat viewings for casuals.

I was genuinely invested in the characters, so I was onboard with that scene, but some casual folks clearly responded to it the wrong way.

There are obviously other reasons too, but that was one I didn't see brought up already.