James Cameron sold off his rights to the Terminator franchise when he was young and hadn't built his name yet. He was just happy to be working on a film he told IGN, and he left the franchise after the second film because he wasn't in control anymore. In his absence, the franchise hasn't exactly done very well. The films and television series that followed have not lived up to what the franchise once was and lead to an opportunity for Cameron to get back some of the control of the property.
When the opportunity arose, Cameron said that he wondered if there was an artist reason to return to the series. Was there a story there for him to tell. A lot of the things that were science fiction at the time he made the films are now either reality or at least being discussed about for the near future. Predator drones are being used in war and artificial intelligence and robot soldiers are not that far out, which leads to the discussion if a robot should have the ability to choose to kill. Something that totally goes against Isaac Asminov's Three Laws of Robotics… just saying.
Cameron thinks that there is still room for a movie to examine those types of themes, but they have to be reshaped to match today's audience and their expectations. Which makes sense that even though science fiction is often set in the future with advanced technology, it always focuses on the social issues of that era. Meaning that they tech needs to continue moving forward while the issues need to be current.