James Cameron makes films so that he can get someone else to finance his real passions. He made Titanic so he'd have an excuse to build a submarine. He fully understands how to make good films, but his objective is to get an excuse to do technically-interesting things in a big-production setting. His most reliable path to that is to make big key-jangling films that do well commercially so that they keep giving him more money to do whatever he wants.
Lucas cares about the art, he just sucks at it. The prequels were his ego project where he thought he knew better than everyone else about how to make Star Wars. Cameron knows how to make art, but it's not what he's after. He just makes the thing that he knows will work so that they keep giving him carte blanche to make new cameras and whatnot. The comparison between the prequels and the Avatar films is reasonable, but I think Lucas and Cameron arrived at those places in different ways.
Cameron never made another Aliens or Terminator or Titanic because he didn't have to. Lucas never made another decent Star Wars movie because he didn't know how.
This is not meant to be lionizing Cameron too much, just that he very clearly knows how to make successful films, and he also very clearly enjoys playing with technology more than he cares about art. Art is no longer a useful vector for his success, so he's not going to waste his time with it.
