Publishers like series novels because if readers like the first one they’ll keep coming back year after year for the latest adventures of your protagonist. That’s why we see so many books labeled something like, “A Martin Kane Mystery.” To be honest, I’m not a fan of many series because so many of them seem to be about serial killers, which is not a topic I enjoy, but they are popular with a huge audience.
Author Jeff Abbott (“Adrenaline”) told the Shelf Awareness blog, “"I'll only do a series if I find a great main character, because basically doing a series means that for the next few years--sometimes more than a few--you're going to be married to that person!"
Of course you also have to come up with at least a thin reason why your protagonist keeps having these experiences (although it never seemed to bother readers of the Miss Marple books or “Murder She Wrote” series that these ladies routinely encountered murders). In the case of Abbott’s series, he wanted to give his protagonist, Sam Capra, a reason to travel the world. He made Capra the owner of a number of bars in different countries.
Finally, your antagonists have to be as strong as your protagonist. Abbott says, “"Criminals with very large aspirations are really fun to write about, and they're also really fun to pit an individual against.”
If you can fulfill these requirements, you might just have an idea for a hit novel series.
(for more tips on how to write a novel, see my book, "Your Writing Coach," published by Nicholas Brealey and available from Amazon and other booksellers.)