What makes a movie franchise a franchise and not just a collection of similar films? Commonalities.
Sometimes it’s just a character by himself that makes it a franchise (Tarzan). At other times a common theme (Final Destination). Sometimes films of a franchise will share a common style if they are all made by the same filmmakers (Jason Bourne), or expand on a singular story (Star Wars). For others, it's a gimmick or hook (Home Alone), or a premise (Planet of the Apes).
The Alien franchise doesn’t necessarily fit into any of these categories; although it has featured all of these traits at one time or another. It started in 1979 with the release of Alien, a sequel came seven years later in Aliens,