Copy-pasting from this Reddit comment:

I think that it is definitely more than one reason. Here’s how I break it down.

1. Avatar is one of the few major franchises that don’t require homework

This is a big one for me, and why I feel like a lot of people enjoy Avatar. It is extremely accessible. Look at our major franchises–Marvel, DC, Jurassic Park, Fast and Furious, Harry Potter, etc. These franchises have been coming out for literal decades and require so much homework and hours of content to watch the newest release. Marvel has made it worse by creating Disney+ shows. If you miss out on a couple Marvel releases, you will be set back from watching the newest release. Avatar is not like that. If you watch the first one, you are good. Simple.

2. Avatar is something new in a crowded market

This point is connected to the first one. In a space where we get the same blockbusters again and again, Avatar is something different to look at.

3. It is not too complicated

People rag on Avatar’s simple story, but the simpleness of Avatar is paramount to its success. It is very easy to follow. It doesn’t demand too much of the audience. The characters are black and white. There is clear good and evil. You root for the relatable family just trying to survive, and root against the evil military baddies. Themes of family, safety, persecution, love, and nature are universal and not beholden to one region.

4. Avatar is four quadrant.

Avatar is the definition of a four quadrant franchise. There is something appealing about it to every demographic, especially after the children characters were introduced. The films do a great job of displaying diversity in ages, without dumbing down the characters either. Everyone can see themselves in at least one character. The characters being blue aliens also help people project themselves onto the characters without the barriers of real world race and politics.

5. It looks pretty, and incentives premium screens

The Avatar films are gorgeous. The Way of Water has the best CGI I have ever seen. Movies are visual mediums, and if a movie looks pretty, then that will be remarked on. People want to see it on premium screens, which costs more. People are also willing to wait for a better screening and sits, which contributes to the low drops it receives week by week.

There may be some other points missing but to me, this is the key five reasons why Avatar did well. All of these points can be attributed to Top Gun: Maverick as well.

Edit: One final thing. Let’s make it a New Years Resolution to ignore fanbases. Fanbases don’t mean anything in the grand scheme of things. Rabid fans have never truly impacted the box office. The true money has always lied in the GENERAL AUDIENCE. Avatar and Top Gun ate big for older people and everyday people who just wanna be entertained during the holidays. It doesn’t matter that you don’t see people cosplaying Na’vi at Comic Con. Fanbases and memes don’t equal box office success. If it did, Morbius would be a success, Henry Cavill would still be Superman, and Blade Runner 2049 would have had a sequel by now.