In the annals of sci-fi movies, there are a ton of different franchises out there. In fact, there are some science fiction films that probably shouldn't have been made into franchises. On the other end of things, there are quite a few sci-fi films that stand alone and were never made into franchises that absolutely should have been. There are a couple of different reasons why that's the case. Sometimes, the movies simply weren't popular enough. Sometimes, the people who wanted to make a film a franchise just couldn't figure out a way to do it.