RoboCop 3 was the last of the mainline RoboCop movies, up until the recent RoboCop remake. That might be because of the fact that even the filmmakers didn't quite know what direction to (blog) go with the story at this point. Or, the PG-13 rating. Or... well, I guess like always it's simply a combination of many factors.
Source: RoboCop 3 Movie Review - Decker Shado