I don’t claim to be a great marketing guru, with reason. When I was younger, after a tour and a half in the Navy, largely as a helicopter search and rescue pilot, I spent a year as an industrial economist, technically a market research analyst for a company that manufactured compressed air valves, regulators, filters, and lubricators for heavy industry, largely automobile manufacturers. I wasn’t a good fit. The next year I got a real estate license, and in that year, I sold two houses, just two very modest dwellings.
Then I started writing science fiction stories, quickly discovering that the few stories I sold didn’t come close to paying the bills. But the writing and economic skills landed me in paid political positions for the next eighteen years, while I wrote and sold SF novels on the side. Those novels paid much better than stories, but not enough to leave the day job, not until I wrote my first fantasy novel – The Magic of Recluce.
When I started getting those first stories published, most of what was selling in the overall speculative fiction field was science fiction, particularly novels by Heinlein, Murray Leinster, Asimov, Simak, Poul Anderson, Arthur C. Clarke. While Lord of the Rings was first published in 1955 in Great Britain, it didn’t appear in the United States until 1965. Despite the fact that Lord of the Rings sold something like 150 million copies, it took a while for overall fantasy book sales to surpass SF sales, but by the mid-1990s, total fantasy sales were definitely eclipsing SF sales.
This trend appears to be continuing. The editors I know say that it’s getting harder and harder for SF novels to be published, while the fastest-growing segment of speculative fiction is Romantasy – fantasy novels with sexual and romance content verging on the pornographic.
Part of the decline in the sales of SF novels is that the wish-fulfillment aspect of those novels gets harder and harder to pull off (if the author wants to stay close to the scientifically accurate), given scientific discoveries over the past few decades. Venus can’t be a tropical planet because it’s a hellhole in reality, and Barsoom can’t really exist, although several authors have gotten around those facts by setting their stories in alternate universes, but that makes those books science-fantasy, rather than SF.
There certainly are exceptions, such as Andy Weir’s The Martian, but they’re getting fewer and fewer. Part of that may be because SF has historically been dominated by male authors writing for male readers, and the reading rates for men have dropped dramatically since the advent of the internet. Whatever the other reasons may be, from what I can see, publishers overall are releasing and selling less hard SF, and even less fantasy that doesn’t have either sex-related romance and heavy action-adventure.
But what do I know?