At one time, when a horror movie was successful it didn't automatically mean that a franchise was born. For decades, Norman Bates remained incarcerated. After Leatherface danced by daybreak in the wake of Sally Hardesty's escape, his chainsaw stayed silent for years.
So even though Michael Myers disappeared from the lawn of the Doyle house after taking six bullets in the chest at the end of Halloween, it didn't necessarily mean that audiences would ever see him again.
Halloween's open ending may look like an obvious, even cynical, sequel set up from today's standpoint but it was simply ambiguous. The only curtain call Michael needed to make was the sound of his breathing playing over the film's final shot of his childhood home.
That said, in 1981 it was hard not to be excited by the prospect of More of the Night He Came Home. The trailer promised to give us more of Dr. Loomis' dogged pursuit of Michael and more of Laurie Strode in peril. Slasher movies were booming and surely this sequel would show the makers of all the Halloween knock-offs how it's really supposed to be done. As it turned out, though, not so much.
Halloween II began the long tradition - celebrating 40 years now! - of fans finding themselves largely disappointed in Halloween sequels.
It also began the cherished tradition of fans continuing to hope, against all logic, that the magic of that first film might one day be recaptured.
The fact that II, even in the hands of Carpenter and Debra Hill, entirely missed the point of Halloween, though, should have been a strong indicator that this ship would never quite be righted. I don't think there's a single franchise that so immediately got off on the wrong foot with its first sequel as Halloween did. All the subsequent mistakes that other Halloween films have made were born from the mistakes of this one. They stumbled on the first try and they've been falling forward ever since.
Some of the Halloween sequels and reboots have been better than others. Some have been pretty nifty in their own right. But none of them have really, truly made a convincing case that Halloween should have ever gone past the first movie.
What Halloween II had that none of the other entries would (because of Halloween II!) is the benefit of the doubt. Once II was released, every future Halloween was forced to live in the shadow of that first disappointment. It will forever be the only sequel where the trailer had fans mostly expecting a treat rather than being wary of a trick.