Science-fiction, save for a few hard SF novels which are too boring to even contemplate, is straight-up fantasy with just a slightly different cover.
Asking why fantasy is more popular than science-fiction is like asking why sports is more popular than football.
I was going to post something similar. I completely agree, but with at least one caveat, stated below. Generally speaking, science fiction is a part of fantasy, just as fantasy is a part of fiction.
From a marketing perspective, though, bookstores in this country often have separate shelves for the two, SF over here and fantasy next to it over there. To the degree that that sort of thing is done, it might make sense to distinguish the two. Apparently, some people get their feathers ruffled when the books start to intermingle.
I also believe that fiction categories should be formulated descriptively as opposed to prescriptively. There is a corner case of works exemplified by Robin Cook's Coma
that aren't routinely shelved with science fiction either (but which in my opinion are SF). Coma
isn't even shelved with fantasy. It's routinely shelved as general fiction, suspense, or thriller. Now, why is that? Well, besides marketing, also probably because there's very little speculative
science, although it all wraps together into an application of medical science which itself is speculative, fictitious, and culturally relevant, in a cautionary tale (which is why I think it's really science fiction). As to what the caveat is, it is that this sort of work seems to dispense with almost all the so-called fantasy elements, and bridge straight across from science fiction to general fiction.