This is about why writers write series.
Reason number 1 is probably once you’ve done all the work (and it is WORK) of creating your world (whatever it is; this can be a contemporary (to her) mystery world like Ngaio Marsh’s series about Detective-Inspector Alleyn, or a near-future mystery world like J.D. Robb’s ‘In Death’ series, it can be romance or SF or fantasy - it’s common across genres) it is simply much easier to continue writing in that world than to create a whole new one for each new book.
Reason number 2, I believe, is that readers like a series. Same as TV viewers do. The soap operas born in the 1950s were not wildly-popular, immensely-profitable, incredibly-long-lived works of storytelling because viewers had nothing better to do. They were all those things because viewers got involved.
J.D. Robb is publishing her 50th novel in the ‘In Death’ series. I have read all of them. I will continue to read them as long as she continues publishing them. The world she created is interesting. It’s evolved a little since she started the series, but not all that much; her near-future is in many ways like the modern world. We will not catch up in technological terms, but the science-fiction elements are not so out-there that they make the books seem more SF than mystery. The through-story, of course - the thing that drives the series - is romance.
It started out being homicide Detective Eve Dallas and the mysterious, formerly-criminal billionaire Roarke. As the series has gone on, there are more and more secondary characters, some of whom have strong romances of their own woven into the ongoing storyline. These books move fast: the characters have only lived through a few years in series time.
A series in romance is almost a must-do. The typical approach is to start with one couple and then write follow-on books featuring friends of one or the other protagonist of the first book. This works whether you’re writing historical or contemporary, because people do have friends, and those friends tend to be at the same life stage. A college student has friends who are other college students. Young professionals, middle-aged divorcees: we’re talking peer groups. Some of those groups are more likely to be single and looking for love, so those are the friend groups most often found in romance series.
A common historical approach is to take a group of men who are friends from public school or university or the military, and write their romances. They’re all around the same age, all likely to be single within the same period of years, and - key in historicals - mobile. Women didn’t have the physical mobility that men had. A group of female friends is trickier to write, for that reason among many others.
Obviously, that’s what happened with me. A single novella with its two and a half protagonists almost immediately spawned a second novella featuring that half-a-protagonist and a new character. Each set of protagonists brings different friends and events into the story universe.
Mine are a little different, in that there is no single common bond. Some of my characters orbit around the (fictional) West Hollywood ballroom dance studio called Shall We Dance. Some are involved with the (fictional) semi-pro dance company called the Underground Cabaret. Some work in various (fictional) law firms, mostly in Century City, a West Los Angeles business district about a mile from West Hollywood. Some work at the (fictional) Hollywood nightclub called Chrome. A group of things I’m working on now are set in Las Vegas.
Some of my characters are created because the story requires a person who does X in Y location. Some are created because a protagonist simply needs someone to talk to. And - as noted above - people have friends. No man (or woman) is an island. It’s an ever-growing network. Every one of the Las Vegas characters came about because of the first Las Vegas character I wrote, Lucas Gutierrez of TODAY, TOMORROW AND FOREVER.
I like what I’m doing with this series, and it’s partly because I don’t feel like I’m reaching for the next story. I’m not looking at one pair of main characters and thinking, what other friend of theirs have we not met yet. My characters appear when I need them. Some of them won’t be the heroes of new books; they are simply people in the world of these stories. But I need them, because these stories are set in a world.
I am currently reading several series written by others. I’m about to read book 5 in the Whyborne & Griffin series by Jordan L. Hawk, I recently finished the last-available new title in KJ Charles’ Lilywhite Boys series, there is the aforementioned J.D. Robb, and there’s the excellent romance novelist Stella Riley (her Rockliffe series is THE BEST Georgian romance I’ve read since Patricia Veryan).
Once you find a world you like, stick with it.