Success of both were unexpected but,
reasons why Avengers made 1.5B ww
1. It was the first superhero Team up of all favourite superheroes of MCU
2. The worldwide recognition of all those major superheroes thanks to the solo films of MCU started with Iron Man in 2008.
3. Popularity of Marvel comics, which're sold worldwide in massive numbers daily.
4. Huge domestic box office gross was expected.
5. Broader appeal especially from families
reasons why Avatar made 2.749B ww
1. It was a movie, a world(Pandora) we've never seen before.
2. Repeat viewings
3. First massive scale 3D release worldwide.
4. As it was the first 3D live action film, it was an experience of the alien world Pandora.
5. Very positive WOM than any other film in history.
6. It was released in winter(December). So any movie get legs, especially a movie like Avatar with an extraordinary WOM.
7. First movie from Jim since TITANIC.