You don't have to do as much research to do a fantasy. Tolkenesque fantasies tend to be set in a pseudo-medeaval world with monsters and magic added. There is the standard fantasy which is much agreed upon and which has a wide audience. There are however many different types of science fiction, many different levels of scientific realism so the audience for science fiction is much more greatly segregated than that for standard fantasy as one swinging a sword in one setting looks pretty much like someone swinging a sword in another.
Also there are parallels between the Star Trek setting and a Tolkenesque Standard fantasy setting. For example what if the Lord of the Rings were set in a Star Trekish science fiction setting. Already Star Trek has races of elves called Vulcans and Romulans, they are both long lived like Elves.