A good science fiction writer not only writes something scientifically plausible (even if it is a little hand-wavey), but also shows us what the implications would be for what might happen if we had technology that we think we want. It usually starts with a "What if...?" scenario, like, "What if we developed submarines and underwater bases, and were able to explore the earth 20,000 leagues under the sea?" One of the reasons that science fiction became an established genre in the 19th century is that technology was progressing at a rapid pace, and had been for some time, so it started to seem prudent to examine where our technology was going, and whether we really wanted to do things like bring dead people back to life.
A thousand years ago, though, technological advancement in the West was pretty damn slow. The Arab empire happened to be hot shit, but aside from some of the stories in the Arabian Nights, they did not give us much one could call science fiction. Going back even further, to ancient Greece and Rome, even though they had scientific advancements that were changing their quality of life, fantasy was the dominant genre for fiction, and the stories they were writing far more often involved the gods and other supernatural things.
So, in the literature of the world for those 2,500 years, there were a few stories written here and there that one might say were more like sci-fi than fantasy. But it didn't become what you'd call an established genre until the 19th century, when you started to have writers contemporary with each other, and critics starting to recognize it as a genre in its own right.
Because I am a nerd, I once had an idea for how one might write a science fiction story set in the ancient world: machines like those used by Archimedes start to see widespread use, improving the quality of life for many people, most prominently by making farming easier and more prosperous. People start neglecting their religious rituals, and their religion in general, thinking that the gods have nothing to offer when machines provide all the prosperity they need. And then the ending of it would somewhat resemble "Godzilla vs. Gundam Wing."
Because I am a nerd, I was also going to write it in Attic Greek, on vellum, and otherwise do everything possible to make it appear that the story had really been written during that time period. Unfortunately, that project is currently taking a back seat to all of my other writing ideas, because who the fuck writes in Attic Greek.