In the traditional publishing model, the writer's customer is the editor, not the reader. In theory, the editor is choosing works that will appeal to readers, that will make the company more money. In practice, many editors came to view themselves not as someone who finds the stories their audience want to read, but as selecting things that will teach their audience what they ought to want to read.
Furthermore, while many editors would say again and again in their submission guidelines and their editorials that they wanted "good" writing, but often what they were looking for was less a matter of objective literary quality than what was fashionable. Whole sub-genres and even styles of narration disappeared from the shelves simply because the literary establishment declared them "out," just as quickly and decisively as narrow ties and lapels would vanish from the clothing racks when apparel industry taste-makers declared them unfashionable this season.
Thus we see the situation where series that were actually popular got killed off merely because someone in the editorial staff disliked them. For instance, in the afterword to the James M. Schmitz collection T'n'T, Telzey and Trigger, Eric Flint notes that "The Symbiotes" was the last Telzey Amberdon story that Schmitz ever wrote, yet he stopped at a point where he had plenty of directions he could take Telzey's future stories, which leaves one wondering what else he might have done with her. Recently, I have learned that Schmitz stopped writing Telzey stories because his principal market, Analog SF, had gotten a new editor who declared the character was no longer welcome.
If established authors with a strong fan base could be put in a position of having to shelve a beloved character (or setting, or whole 'verse) simply because of changing editorial tastes, the effect on aspiring and beginning writers was much more intense. When one's only path to publication that would be taken seriously by readers was to win the approval of the editor who functioned as gatekeeper to publication, writers became frantic editor-pleasers. Much effort went into trying to figure out what editors really wanted, divining what might be the actual editorial desires hiding behind the words of their official pronouncements, not to mention trying to figure out how to gain that "golden handshake" that would get your manuscripts out of the slushpile where first readers are instructed to read to reject, and get actual consideration. And given that editorial tastes could turn on a dime, there were plenty of projects begun, then abandoned as soon as editors started saying they'd seen "enough" or "too much" of a given theme or story element.
So if you did a project just for yourself, just because you loved the idea, it really would be just for yourself. You'd set forth with the knowledge that it would likely go straight to the proverbial dresser drawer, at most to be shared privately with a few like-minded friends, but never to go anywhere further for the simple reason that no editor would be interested in it -- and self-publishing was vanity publishing, and the kiss of death to the reputation of any professional writer. Maybe, just maybe, some day your heirs would come across it in a musty corner and connect your old agent with the discovery of a never-before-seen manuscript of yours, but more likely it would molder to illegibility and finally be tossed out with the trash by heirs tasked with handling the estate.
But with indie publishing being respectable, and even quite remunerative financially, it's possible to write a project that's just for your own enjoyment, and when you're done, hand it to a few trusted readers to see if they think there's any interest in it. And if they are excited about it, you can decide how much money you want to put into preparing it for indie publication, and how you're going to go about presenting it to your fan base. Because now you're selling to your actual readers, not to someone who is supposed to be an intermediary and gateway, but often functions more as a taste-maker, deciding what people ought to get excited about.