I think part of it is definitely things going in cycles, as others have said. And part of it seems to me to be that writing things for SF can be a bit more difficult than doing the same for fantasy. My perception is probably wrong, as there's no shortage of things to borrow from for inspiration, so there shouldn't be much difficulty in explaining things with technology instead of magic for whatever space adventure one wishes to do.
It probably does also depend on what sort of scifi one desires. If I saw something good, I'd probably give it a try, although I don't think I have the right disposition to actually run a game myself here. I'm more suited to GMing things live.
I do wonder, how much does SF feel like it gets bogged down in the technical details, or otherwise feeling less accessible or the like, compared to fantasy? I haven't even seen much for interesting things in stuff like a giant mecha genre, and such things are usually quite popular.
Part of the problem with Sci-Fi is the execution of the technical details. Unless you INTEND to include fantasy elements (which ultimately result in a divergence into science fantasy), nothing can simply be explained away with "the FUTURE" unless you deliberately avoid discussing certain things in detail. The advantage of writing in fantasy universes is that magic, while it must have rules, can basically explain anything, and worlds don't have to make sense when compared to the real world because that's part of the point of some fantasies. Science fiction, however, is somewhat rooted in reality, and sometimes almost wholly rooted in reality. The fiction part comes in the form of technologies that are not fully explored or developed in reality being commonplace or fully functioning in the world of fiction (FTL travel; instantaneous transport; perfect, compact nuclear fusion, etc.).
As I said previously, what can make or break a science fiction story is how well these things are explained. Once again returning to fantasy, magic must either have concise, unbreakable rules that can be easily followed by the reader, therefore allowing them to immerse themselves in the action without being confused
by the action; or magic must have few or no rules, adding to the wonder of the story, while not being used as a
deus ex machina (this is essentially
Sanderson's First Law of Magic). While this law does apply in a way to science fiction, the game changes in subtle, yet important ways. Technology, unlike magic, is a material thing. It functions in accordance with natural laws that humanity understands to be universal constants. For example, an object simply cannot reach or exceed light speed without some form of manipulation that is currently beyond the capabilities of mankind. Soft science fiction escapes these limitations by simply resorting to quick explanations that provide a backdrop for the action (Star Trek, Star Wars, Mass Effect; each relies on a similar excuse for why interstellar is possible, that being certain fuels, engines, or accelerators that push ships into a separate dimension or beyond light speed). Hard science fiction, however, rarely approaches the problem of FTL simply because the exposition involved would slow the story dramatically (the Martian: in this story, the science takes precedence over the fiction. Real physical phenomena are used to explain how more rapid travel may be achieved in space, how plants could be grown on desolate Mars, and how semi-normal artificial gravity may exist whilst traveling in space. Nowhere, however, are more fantastical concepts like FTL explored).
In summary, science fiction (Space Opera, Mecha, etc.) is more difficult to get into than other genres simply because it is SCIENCE fiction. There is no way to simply disregard the physics of something without (A) leaving the universe shallow; (B) bringing in elements that proportionately increase the fiction at the detriment of the science, until the whole thing eventually becomes a science fantasy, or a fantasy altogether; or (C) leave the writer or writers too much liberty with what they can do with the universe (in an RP, this is in regards to the GM/player dynamic wherein the player wants to go in one direction, whereas the GM's semi-formulated vision is in opposition). Many people simply aren't ready to go to the trouble of understanding the science of a story, don't care about the science of a story, or don't understand the science of a story.
