I remember reading, a few years ago, Amazon reviews of the 1990 William Gibson/Bruce Sterling novel "The Difference Engine". Apparently most people expected a normal novel, just with a "steampunk" setting, so naturally they were disappointed and complained about the book being confusing. That's because it's a cyberpunk novel. Which is a literary genre, not merely a setting like steampunk. (The latter term didn't even exist when the book came out.)
I remember Stanisław Lem (an SF author well-known outside the English speaking world) said approximate this about historical novels: Historical novels have the advantage of depth, they can reference a world that is much more complex than required for their plot, they can set themselves in the deep complexity of actual history -- whereas fantasy and sci-fi books must always rely on their own made-up world, which almost necessarily looks flat and shallow in comparison, even if it seems spectacular on the surface.
I really came to understand this when I read Umberto Eco's "The Name of the Rose". All the historical details are so intricate that they are almost impossible to match by a novelist writing about a fantasy world or the far future.
This is, perhaps, also why The Lord of the Rings is such a great fantasy story, and why most other fantasy stories fall short in comparison: Tolkien didn't just write a novel. He invented a fictional language first, then an elaborate fictional history around it, and the Lord of the Rings is really just a small part of this story near the end. When reading the book, you constantly read allusions to "historical details" about things that happened thousands of years ago in Valinor, Beleriand, Númenor, in certain ancient wars etc. These "superfluous details" are occasionally hard to understand (except if you read Tolkien's posthumous "Silmarillion", which his son compiled from fragments) but they approximate something like the depth that usually only a historical novel can achieve.