The last time you heard that there was going to be a sequel to one of your favorite movies, did you jump with joy, or think "please don't fuck it up"? For me it's definitely the latter the vast majority of the time. Series and sequels have a horrendous track record. Far too many sequels are made to material that was only ever meant to stand alone, and that's not even the issue I want to address.
The majority of art is not good. This is just a universal truth. It's easy to throw something together without thought or effort and wind up with a mess. Arranging elements intelligently and creatively requires a lot more skill, planning, and persistence. Even with all of those factors present, works of art are not always successful.
If you've ever studied film-making, you know that pre-production is a such a big process that there are entire jobs within it that don't extend to production. The success rate of all art goes way down with a lack of planning. The number of projects that a writer actually finishes is about 1% of how many they started, because spontaneous inspiration leads to immediate creation, but most of the time it turns out that the idea didn't really have that much potential. When this same process extends to published works, it often leads to a betrayal of the consumer.
Ideally, when the creators of a series are not sure that they will get the financial backing to finish it, they should make sure that the first installment has a satisfactory ending, even if it doesn't tie up every loose end. Then, when it becomes more clear that the series is slated to finish, it can be structured around the eventual ending and its necessary setup. Not only is the ending one of the most important components of a story, as the last impression with which to give the audience a sense of closure, it also informs the rest of the story, because taking the necessary steps on the path to the ending gives the story direction.
This is not what happens most of the time. Works that are intended to be the beginning of a series usually end with a cliffhanger, thanks to the false assumption that audiences are only interested in knowing what happens next regardless of the quality of the work. Then installments are structured as an ongoing story rather than one to be completed, mainly due to a major financial incentive to make as many installments as possible. Without knowing the ending ahead of time, at least of a saga if not the whole story, it is impossible to set up proper buildup and payoff. Because writers so often have no intention of letting the story end, they often do not even know what the ending will be until it hits them in the face. This is most evident in TV sitcoms which often have a sudden awkward attempt at catharsis in the final episode, which clashes with the show's tone and generally doesn't work because there was no attempt to get ready for an ending.
As if serial storytelling didn't have enough inherent challenges, sometimes the publishers get involved on the micro-level, and that very rarely ends well. Since publishers by inclination are people who know money and not art, they usually do not understand what makes the story work. The worst offenses are direct changes to the writing, such as preventing popular characters from dying. The obligatory terrible sports analogy is a team's financial manager making the decision that practicing passes is a waste of time and every player should just go for the goal selfishly. Publishers also have the power to determine how much material the artist has to work with. The third Spider Man movieF had a clusterfuck of three different villains because the director originally wanted to make three movies instead of one. Apparently the superhero movie that brought superhero movies into the mainstream was still popular, so it got an abrupt reboot with a much less charismatic Peter Parker who also happens to be an asshole in real life.
The Harry Potter movies are examples of both too short and too long. The shortest movie in the series is adapted from the longest book, but the last book (which is arguably the shortest because even though its page count is not the lowest, it's full of filler) was adapted into two separate movies titled as part 1 & 2. Apparently the studio forgot that they were making adaptations of a finite number of books, and once they reached the bottleneck (or rather bottlecap) of "there aren't any more books after this", desperately searched for any excuse to make at least one additional film and prolong the profits. The same thing then happened to Twilight and Lord of the Rings.
Money people also have a habit of forgetting that art is created by artists. One of the most common downfalls of a TV series is firing the head writer. You know, just the person with all of the visionary direction and ideas. CommunityB is the most recent example I know of, losing its creator after the third season. The fourth season still had obvious influence from him, but once the remaining team ran out of his ideas, the fifth season was clearly on a downward path, and then the show was canceled. It did get picked up by another company however, so the story doesn't end with asteroids killing all of humanity. A less recent, but more evil example comes from Disney (you knew after reading "evil" that "Disney" couldn't be far way), who fired the head writer of GargoylesD because they couldn't give him the larger third season budget he wanted, then gave his replacement everything he asked for.
One of the praises shared by proponents of anime is that Japanese culture makes businesses much more loyal to their customers because there is great shame in betrayal or failure. Remember when the Playstation Network had a huge security breach? The CEO of Sony had to get on camera to apologize, not "we regret that some unfortunate events occurred" but actually "we're sorry we fucked up." Ironically, some of the most popular anime series do in fact run on with no ending in sight, but there is some truth to the culture-based claims. As far as art goes, Death NoteF is the best thing ever to come out of Japan. It has 37 episodes, and then it's done; not because it was canceled or had some legal battle but because the story concluded. Before you say "well that's just the best of the best", consider exhibit B: Lucky starD, the most popular show in the vapid slice-of-life genre, comparable to sitcoms like FriendsD or The SimpsonsC. There could have been an episode for each of the characters' 1461 days of high school; instead there are 24. It's also apparent that there was some advance planning, because characters who are not introduced until the 16th episode can be seen in the opening credits of the first.
Of course, we cannot blame every problem on money. Not all writers are good writers. Some would rather not even try to give their stories proper endings, and instead "leave it up to interpretation". The SopranosD is the most blatant example of a non-ending in recent memory. At the end of a mediocre final season, full of so many character deaths that they must be either unjustified in happening or unjustly delayed due to main character immunity, the final scene literally jump cuts to pure silence and darkness. Then the video lingers there for a solid 20 seconds before rolling the credits, to trick viewers into thinking the ending was there but missed due to poor satellite reception. Truly a prank leaving Andy Kaufman applauding far away from his fake grave. Unwavering fans of the show have tried to justify the blank screen ending by saying that "cut to black" was foreshadowed earlier in the season, which is sort of like saying that assault justifies battery.
Stories that were canceled, ended but didn't conclude, or had endings that are just plain silly all necessitate replacing canon with headcanon. Think of some mental fan-fiction that ends the story properly and declare that to be the truth. The guy from the bathroom has snuck in a gun and shoots Tony Soprano in the face. The end. Sam Becket meets God? No, he just has a really tough job, finishes it with a sigh of relief, accepts his role as a do-gooder, and continues leaping with no definite retirement. For any "but it was all a dream" ending, just purge that part from memory. There are many great stories that do have proper conclusions, but they are exceedingly rare, so be ready to Bring Your Own Ending.