Fantasy in tabletop RPGs are piss-easy to do. Make characters. Work together to explore ruins, fight monsters, get treasure. It's so piss-easy that they got automated into videogames within a few years of their invention. For all the pretensions of everyone and their mother--especially the SJWs--this remains the core of the category's gameplay because it reliably works. Everything beyond that gets away from the core appeal of the genre, which is why fantasy RPGs that are not D&D or its clones/knockoffs have sweet fuck-all for presence, influence, or power- and yes, that does include videogames. (Where World of Warcraft is to MMOs what D&D is to TRPGs, for example.)
Science Fiction's second-place status in tabletop RPGs stems from this lack of a so-simple-a-retarded-rhesus-monkey-can-do-it default structure. That doesn't mean it doesn't have equally strong default assumptions that do, in fact, serve to provide some structure to gameplay. (They also show why you can could the number of successful SF TRPGs on one hand and have digits left over.)
- The players create characters. They have adventures.
- Their adventures occur in a space-fairing setting.
- Their space-fairing is under their control, using a ship that they control.
- All characters are part of the crew.
The first SF TRPG, Traveller, codified these into the structure. The licensed Star Trek and Star Wars games make good use of them. There are very few exceptions, largely because those exceptions only frustrated one of those expectations (and enjoyed lesser success accordingly) instead of most or all of them- or they weren't called "science fiction". By their behavior over the decades, players revealed their preference that this is "Science Fiction". This same thing, "revealed preference", is how we know that fantasy is similarly bound. (And why settings like Tekumel and games like Ars Magica remain so.)
You want to know why so many TRPGs rot on the trash pile? Because they neither define their own category (e.g. Call of Cthulhu, Vampire: The Masquerade, Champions), nor surpass the reigning champion of the category by fulfilling the revealed preferences better than the incumbent. (This is where videogames have an advantage; by the necessity of having to bundle setting and campaign with ruleset, it becomes easier to successfully break out of known boundaries to attain success- at the cost of confining play experience to strictly and solely what you provide in the sale.)
It's been over 40 years since tabletop RPGs got invented. We know by now what works, what doesn't, what players want from this medium and what they don't. You're not in the business of selling books, boxed sets, pamphlets, etc.; you're in the business of selling virtual life-experience opportunities to tinker-minded hobbyists and their friends who come along for the ride. The games are not the value; they're the carrier. The value is in providing goods and services that allow users to connect and play what they what- not what you want.
That means that there is limited liminal space within a given category of TRPG, enough for one dominant property and a handful (at best) of also-rans whose sole justification for existence is to keep the top dog honest and hungry. (This same dynamic is at work with MMORPGs.) If you want to succeed, then you (a) take a slot as #1 or #2, (b) stake out your space to dominate, or (c) sell tools people can use in whatever category they want (that free resources online don't already do just as well or better for free; stop trying to compete with Discord).
If you can't do that, then GET. THE. FUCK. OUT! No, I don't care if you've been at this since the late 70s; you don't own the market, so you're not entitled to be here. Justify your existence or get run out by those that can and do. If that means the entire tabletop RPG market disappears, I don't care. My money is far too precious to waste on that which can't justify itself- and, to date, there is no tabletop RPG that does.