Why Avatar was so successful?

3D technologies were already present but for a long period of time no one was producing them. The fact that Avatar was the first 3D movie since a long time gave it a very powerful marketing Edge.

The technologies used to make Avatar were brand new that no 3D movie ever had them before. The movie wasn't just the first 3D movie since a long time but it was also the first one to use very advanced 3D technologies.

The plot was very interesting that it appealed to a wide range of audience. Unlike movies that appeal to certain types of people Avatar appealed to broad audience and this is one of the reasons it got so popular.

Avatar movie was about weak nations that fight against the oppression of strong ones. Many people saw a resemblance between the movie events and real life politics. This lead to controversy which made the movie more popular. (See Why controversy can make things popular?)

People got to science fiction movies to escape from real life problems. Because the escapism factor was so high in the movie avatar so many people liked it and spread the message.

People talk about things that charge them with emotions. Because the plot was very emotionally charge people kept talking about the movie thus making the word of mouth reach more and more people.

The movie had many important moral values that it tried to pass to people. This made the movie become more than a shallow action movie and can it a bigger meaning.

The human mind always gets attracted to novelty. The Avatar movie had so many novel things that it was too appealing to the mind. The movie took place in a different world, had aliens and so many strange creatures that people have never seen before. (See Why novel and bizarre things catch your attention?)

The Avatar movie was a PG-13 movie. This made the movie more appealing to families who didn't mind taking their children with them. Because the movie targeted broader audience it became even more popular.

James Cameron the director of the movie was the one who directed Titanic, Terminator 2, Aliens, True Lies, and The Abyss. So many people went to see what James Cameron is up to in his new movie.

Some people believe that there were little competition at the time where Avatar was released. And while the movie was produced very well it could have been helped by the fact that little competition was present.

The movie had a strong environmental message which appealed to people who care about the environment. The fact that green thinking is becoming more and more popular could have helped some people find the movie more appealing.