1. Cameron has made 2 of the best sequels of all time.
2. Cameron has passion for the project
3. He has hired various professional writers to make sure his next films have a complete and cohesive story in order to not fail like so many other sequels.
4. Now that the first film is out of the way, he can focus on more complex elements, Star Wars: A New Hope was also very simple and derivative before Empire took it to a whole new level.
5. Cameron himself has promised a "Crescendo", with each film being better than the previous one.
6. They're actually taking time to get the plot and world right.
Many other reasons could be stated, but i would have to be here all day, long short story, never underestimate James Cameron.