There’s an ongoing trend in SF and Fantasy (particularly in Fantasy genres) for books to get longer and for series to be longer. Many authors like it that way, many readers like it that way… but why? We can all harken back to the golden age of SF when many authors wrote stand-alone novels… or can we? Asimov’s Foundation series began as a trilogy and eventually became not just seven novels, but also the spiritual sequels to his Robot and Empire series. Heinlein often wrote stand-alone novels, but later on in his life he also tied them together, linking older books together through time travel and having old characters act as mentors to other characters. Granted, there are and continue to be many authors who write stand-alone books, where the story arc follows just the one set of characters through one book. But currently, this is becoming less the norm and more the exception.
In fact, as you look further back than the “Golden Age” you might notice something of a trend. Jules Verne, Robert Howard, Edgar Rice Burroughs, and Doc EE Smith were some of the founders of Science Fiction as we know it. They often wrote in serialized formats, either for magazines or pulp markets. Why, you might ask? Because that was the way to draw and keep an audience. As stories go on, the readers become more and more attached to the characters and invested in the universe. For an author, that is exactly where you want them to be. Reading is an investment of time and money… and if the reader has already invested time and money into your characters and story, then they are more likely to continue, particularly when money is tight.
The same holds true today and in more forms of media than that of books and ebooks. Movies are increasingly becoming either series or reboots, the former to build “bigger” story arcs and delve into characterization that you can’t accomplish in a mere ninety minutes. The latter is a coldly calculated rationalization that name recognition and recycling ideas is a way to get people to put their butts in the theater seats.
Video games have become franchise sequels or like World of Warcraft or Eve Online, become longer, with ever more added content. This is all a means to the same end: they know that the consumer has invested time and money into their products, they want to get a bit more of it to continue the draw.
Coming back to the original statement, why is this so prevalent in SF and Fantasy? The easy answer is that readers want it. They like seeing more of their familiar and loved (or hated, see Game of Thrones) characters. The investment of learning a new author’s universe is paid back by the enjoyment of continuing adventures within that universe… as long as it doesn’t change too much. Reading is an escape, a means of enjoyment, and we want that escape to continue as long as it can. That is why we like series and broad, epic books. Because once we’ve invested the time into that story, we don’t want it to end until we’re ready for it… and one book is seldom enough.