Stargazer’s World asked the question, why is the sci-fi genre second to fantasy in RPGs? I began to write a comment but it got too long so here it is. I suggest you read his excellent discourse on the subject first.
First off I do believe Traveller was in fact the flagship, go-to system back in the day. I remember it having a huge following (and it still does) and in the eyes of everyone I knew (so this may be subjective) it was The Sci-Fi Game. Eventually it lost that position. Why? For some of the same reasons why sci-fi RPGs are not on the same level as fantasy. And here are some of those reasons...
-Diversity/Niche. Cyberpunk, superheroes, space opera, hard science, near future, psionics, no psionics, multiple worlds, space combat, etc. These all fall under sci-fi RPGs but they are distinctive enough to garner their own following. And with that diversity there is a fragmentation of interest and purchasers. Fantasy tends to fall into the same general genre, medieval society with spells. Diversity in sci-fi tends to focus on the differences of setting, while diversity in fantasy tends to focus on diversity of rules. Thereby sci-fi games are divergent from each other while fantasy games are less so.
-Too close to reality. Most players want to get away from the normal, the every day. For some of those people sci-fi is too close to what they do everyday. Fantasy allows for the, well, fantastic. In sci-fi games you know the rules of nature. You can’t breathe in the vacuum of space, lasers and bullets will kill you, etc. In fantasy a spell will allow you to breathe in any environment or weapons to bounce off your magical shield. Sci-fi follows real world realities, fantasy does not. For some, it’s that extra step away from reality that they are looking for.
-Math is hard. By this I mean that because sci-fi is based on reality it “requires” the GM to follow those rules. Most GMs don’t know the rules of science, at least not to the degree running a sci-fi games requires. In fantasy, if a GM wants a menace to threaten the characters he can write it off as magic. In sci-fi, he has to come up with a realistic explanation or the setting and his game will lose cohesion. This is further compounded if a GM has a player who is science-savvy.
-Available resources. This pertains to the resources available to the player characters.. Fantasy has built in limitations to the setting that sci-fi can easily bypass. In a fantasy game players use horses to get around, keeping their movement options limited. In sci-fi, they could have access to a ship that could cover the same distance in a fraction of the time thus negating any planned overland encounters. Sci-fi games present another level of difficulty for a GM as they need to account for advanced technology disrupting an adventure.
-History. Fantasy games tend to be based on real-world cultures that people are familiar with. It is easier to gain a basic understanding of the setting. Sci-fi deals with things that do not exist yet, they are completely imaginary (while at the same time being bound by “reality”). For some people, they would prefer what they are familiar with in a setting than trying to wrap their head around a multitude of unknowns.
-800 lb Gorilla. Most players came into the niche of RPGs through D&D or some other fantasy game. It is what they know the best and are the most comfortable with, if for no other reason than that is what they started with. As a subjective example, my wife had been running a sci-fi version of Gamma World (heavy on the science, light on the mutations/fantasy). After that 3 year campaign ended, there were several players who asked for a fantasy game, any type of fantasy game.
Obviously, not all these apply to everyone and every group. But combined together it provides some reasons why sci-fi is not embraced to the same degree as straight forward fantasy. However, I also have a theory that sci-fi is not as less played as it first appears to be. Yes, there is no flagship sci-fi game that is dominant in the same way D&D is. But if you were to combine all the different genres that fall under the title of sci-fi, I believe there would be roughly the same amount of games being run as fantasy, they are just spread out over a wider range of game systems, whereas fantasy is consolidated into a few (though I believe we are seeing a true trend away from that consolidation due to the internet).