1: There is a statistical element to the answer. Simply, there is a reversion to the mean.
The proportion of movies which make a mark is quite small. Most movies remain unknown and most of the ones which become known are quickly forgotten. Even most of the ones people remember are “meh”. The movies whose sequels people look forward to are outliers.
If 1% of movies are so good that people look forward to their sequel (1% being generous) then the odds that two movies, back to back, will be of that quality are 1/10 000. We expect the sequel to be as good as the first because it follows the first but it has about the same chance of being stellar as the first, which is to say, not very much. We are therefore disappointed because the quality reverts to the mean.
2: Another factor is that the makers of the movie face an almost impossible challenge. They have to do the same but different.
When a movie is seen as stellar, a large part of that is that is does something new. The novelty is an important cause of the popularity. Think of zombie movies. The first time you saw one, it had more effect on you than the 50th time.
Yet a sequel has to maintain continuity if it is to be a sequel. To be considered a successful sequel, it must resemble the first movie yet be as novel as the first movie. This is quite difficult to pull off.
The few who did it:
The Lord of the ring and Star Wars (first series, if it needs to be said), succeeded because the sequels are not actually sequels. The first movie is the first act, the second movie is the second act and the third movie is the third act. They’re not sequels, they’re segments of one very long, continuous movie.
Aliens succeeded in doing the same but different. The second movie kept elements of the first such as the alien and the sense of isolation. Yet it changed movie style. Instead of being a suspense/slow horror movie with a monster-that-eats-them-one-by-one, it was a fast-paced action movie with echoes from the Vietnam war.
Another way sequels can work is if they’re formulatic - like the James Bond films were before the last two. If the main character(s) remain unchanged from episode to episode, starting and ending the movie in the exact same place, both in terms of plot and in terms of character development, then you can theoretically make as many as you want. The problem with that is that you can’t have all that much depth, as by necessity the challenges the protagonists face are external rather than internal, and that you pretty much know how they’re going to end.
I feel that a major reason not noted in your OP is that at least some aspect of writing a sequel involves taking what was cool about the first movie and incorporating into the second. The more you tie your hands with ‘we have to include this gag, and that character, and the other dialog,’ the harder you make it for the screen writers to craft a story. And it only gets worse with multiple movies. You can definitley see it in the Oceans movies, and it’s really egregious in the Star Wars prequels, the droids especially.
Fundamentally, the problem is that you came up with a good story, and then used it all up for the first movie. This includes significant plot and character development that is now complete. The bad guy has been killed. The good guy has overcome his crippling fear of whatever and is now a Better Man.
So that’s movie number 1. It ends up doing great in the box office and the bean counters come to you asking for more. So now you have to come up with a whole new story using the same setting and the same characters, who have to go through a whole new arc without shitting all over the first movie.
Now, all this is doable, as films like “Empire Strikes Back” and “X2” have demonstrated. But it’s really, really hard. Even talented filmmakers can fall prey to the difficulty of following up a strong first movie - consider such recent mediocrities as “Iron Man 2” or the Pirates of the Caribbean sequels.
(This doesn’t apply to films that were conceived as multi-installment arcs, like “Lord of the Rings,” obviously. Those types of sequels come with their own potential pitfalls.)
The problem with sequels is that it can go one of two ways:
It can be a rehash of the original, which will upset a certain segment of the audience looking for creativity in situations, characters and storytelling.
It can be an entirely new film, which will upset a certain segment of the population looking for some tie in to the original.
Either way, the adage of not pleasing everyone all the time is entirely true here.
Then add in another variable: a different team is making it. The director might be different, the lead might be different, the editing team might be different, and you’ve got a film that film that can’t capture the magic of the original because that wasn’t the team who created the original.
Even with the same team, perhaps they haven’t actually sat down to think about what it is that made the first movie so special. Or, worse, they think it’s X when any fan could tell you that it’s really Y and so what they’re trying to “recreate” is something that no one cares about.
It seems that many sequels do use the “more of the same” approach, which can be passable sometimes, but the better sequels (IMO) expand on and continue a story that seemed to be left unfinished. So a movie like, say, A Nightmare on Elm Street really wasn’t made better for having sequels, but I think that **[REC]2 **did a decent job of expanding the story and adding more elements and twists. Sure, they might have been not so unexpected, but it didn’t feel like I was watching [REC] just with different actors.
These types of movies tend to be, by nature, really ambitious projects. You only get the go-ahead from a studio to make a trilogy (or even duology) if you have a track record of financial success. And unfortunately, success can breed complacency and ego.
Take the Star Wars prequels. George Lucas didn’t even need a backing studio for those; he had enough cash that he could finance the prequels all on his own. But because he had become such a hugely influential and wealthy man in the industry, when it came time for him to get down to the business of actually, y’know, making the movies, he no longer had anybody around him willing to criticize the work he was doing at any level. Couple that with the fact that he was never a great writer or director of actors to begin with, and you have the first components of the shit sandwich that the prequel trilogy ended up being.
(I should mention that “The Lord of the Rings” is my personal benchmark for a trilogy that came out near-perfect. Jackson took on one of the riskiest projects in cinematic history and succeeded beyond what anyone could have possibly anticipated. But LOTR is a special case.)
I’m reminded of a comment I heard about video games, which seems to apply somewhat to movies as well. That one reason why the same guys that produce the great game (movie) X then turn around and produce the awful sequel Y, is because they aren’t the same people. Something like a game or movie is created by a large number of people, and the sequel is likely to be made by different people. That’s true of a lesser degree for movies than games - they tend to reuse important actors and so forth - but they certainly don’t normally hire the same exact group of people to make movie after movie. So it’s not surprising if a different bunch of people produces a different quality of product. But people look at the big names; the lead actors, the director, the producer, the studio; and think of it as being made by the same people when it’s only some of the same people.
Just wanted to add that the big cinematic showstopper in Christopher Reeve’s first outing as Superman – circling the globe so fast as to go back in time and make things right after all – was originally scripted as the finale for the already-planned Superman II: a feat astonishing enough to top his impressive exploits in the first movie. Of course, they eventually realized there might not be a sequel if the first film underwhelms, and so decided to crank the dials up to eleven for the debut after all.
This is the key for me as to why sequels often fail. The first movie is filled with character devopment, discovery, and improvement. Part of the reason the first movie was good was because you didn’t know what was going to happen. There was more anticipation and suspense. You didn’t know how people would react. But in the sequel you know so much more about the character. You don’t have the pleasant satisfaction of discovering a character’s personality. So a big part of what made the first movie good is no longer available in the sequel.
There’s also typically more movie-craft going on in the original. POTC 1 hit a ton of high notes, with perfect timing, but the sequels tend to severely rush the pace, which means scenes don’t get much of a chance to breathe, and thus they fall flat. Think of Capt. Jack’s entrance to Port Royale, or the dialogue between Elizabeth and Barbossa-not to mention how the truth of their condition as it is revealed to her is slowly built up to, and not rushed, and is the better for it.
Even in LOTR the last two movies are often missing these elements, BecausePJhadToCramEverythingHePossiblyCould into ever-shrinking time budgets (and there’s 2-3 plots going on at once at any given point in the last 2 films, leaving very little room for such things).