Chuck Wendig has a very good post on why he's electing from here on out to write only standalone works, not works in a series. He cites reasons that are a mix of personal/aesthetic, and economic/practical: paper prices are rising; subsequent installments in a series don't sell as well; and then this. It's two of his numbered reasons, and the summation of his reasons:
Writing a series is depressing. It just is. By the time you’re writing books two and three (or beyond), you’ve seen the diminishing returns, the reduced support, the general “farty slow leak of the balloon as it orbits the room” vibe. And that’s a bummer. This is not the most important reason, but also, in many ways, it absolutely is.
... If that first book really doesn’t work, you’re committed to the series anyway. And if you’re not in love with the series, you’re still committed to three books. And that can be… three or more years of your life. A series can be a ditch you drop your tire into and can’t quite drive out of until a good ways down the road. Which can be frustrating and difficult.
... I find them [books in a series] tricky and sort of sad to write, and at this point I’m not intending to write any — sequels, maybe, if a book does really well and there’s a story reason to write a followup. So, you have both narrative purpose and sales numbers there to support more. But even then: I’d hesitate. Because new things are shiny and for better or worse, everyone likes the shiny.
Note the distinction between books in a series, and sequels. A sequel is when you write something, finish it, and then after the fact look at it and say, "Hm, remaining untapped possibilities here!" A series is when you have a larger plan delivered in increments. I've toyed with the idea of sequels in the past but never felt like anything I did needed a follow-up, and the closest I ever came to a planned series was all of Flight Of The Vajra, which I just smooshed into a single volume anyway.
One of the creative reasons he doesn't cite, but which comes up for me often, is how a series always feels to me like a failure of framing. What part of this story is most worth telling and why? Given that I'm going to spend a year or more on this thing, I'd like to ensure that year is well-spent, and that I don't have to keep committing myself over and over again to what amounts to describing different sides of the same polyhedron. Nobody should be in the business of "telling lies for fun and profit" (as Lawrence Block put it) and not enjoy it.
I agree 110% with his sense of how writing these things is depressing. It starts as a way to share a big vision with the world, and it turns into an Oregon Trail through hell — especially if there is the nonzero chance your publisher could just pull the plug partway through the effort and leave you and your readers stranded. I have conversed with other editors in the industry who have witnessed this gruesomeness in action, and they hate it too, but they know why it happens. Publishing is a crapshoot, and sometimes your dice come up snake eyes and you want to cut your losses before you pawn your jacket and watch.
Back to framing again. I think one of the benefits of thinking about work in a standalone fashion is it forces you to think about what stories, or what aspects of a story, are best told as a standalone work of reasonable length. I know I have been far too enamored of using my freedom as a self-published author to write works that were anywhere from 50% to 100% longer than they really needed to be, and I'm trying to address that going forward. If nothing else it makes the editing and production process less burdensome! But one thing I've always tried to do is figure out what exact circle to draw in the universe of that story that encompasses what needs to be the story itself. It helps to make that circle small, but not airless. And one common issue I see with many relatively new authors is struggling with how big, or how small, to make that circle. When presented with one maximalist example for emulation after another, they have a hard time believing anything but the maximalist approach is valid. Maybe having a few less maxis and a few more minis, or at least demis and semis, will help.