In addition to the price of 3D tickets in most places, something Avatar had to its advantage was that it was more of a theme park ride than a movie. It was all about taking in a big CGI spectacle as it happened, which is part of why so many people saw it several times in theaters. With a movie that's more about story or characters, even if it's really good, fewer people are going to go see it again and again and again.
If it's more about story and characters, people who like it will probably see it once or twice, and then wait for home media. With something like Avatar that's so heavily focused on the spectacle though, for most of the audience, the added spectacle of seeing it theatrically in 3D is a big part of the experience; they're more likely to see it several times in theaters, because it loses so much in a home viewing.
Big spectacle movies also tend to do well in a wider range of markets. When people want more story driven films, they're more likely to consider more local fare. With big spectacle stuff, it's just a matter of who's making the most spectacle-y stuff, and that mostly happens to be American popcorn movies at the moment. Couple that with several large markets really opening up within the last decade (such as China, among others). It's how Furious 7 managed to make over a billion dollars in the foreign market, while not cracking the Top 30 in the domestic market.
Personally, when I saw Avatar, I was bored out of my mind. It was very much "Really?
This is what people are going so crazy for?". I'm definitely rooting for Star Wars to beat it.
[pedantic]Also, the "Domestic" market is Canada and the US, not just the US.
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