Simple: it was a very well-made movie that appealed to pretty much every demographic as well as a 3D and technical showcase of the new style. While the core premise was by no means original, the themes of the story were considered and still are considered pretty universal. There's a reason Dances with wolves, Pocahontas, ferngully, and so many other films follow similar structures with different settings and styles. 

I see a lot of hate about how unoriginal the movie is - and it is - but in my time as a writer and in my experiences as a film student/critic I've learned that actual quality is more about execution than intent. James Cameron is a master filmmaker adapting simple, timeless stories into something digestible for a wider, worldwide audience. He knows his strengths and he plays to them very, VERY well. People can bitch about it being pocahontas in space or Dances With Na'Vi all they want, but the director released two back-to-back worldwide megahits that broke all the worldwide box office records back to back. there's a REASON his movies do so well. 

Those extra weeks didn't really help Hi, Mom's box office totals. I see why most movies have relatively short theater runs in China because all their box offices there are extremely front loaded.

Terminator 2 was clearly Cameron's best film, Avatar I didn't understand.