Some idle wondering here. While there are plenty of excellent and successful RPGs in the sci-fi, contemporary, horror, western, cyberpunk, military, superheroes, and other genres, the big daddy of RPG gaming genres tends to be fantasy.
Why do swords and spells tend to be more popular than spaceships and lasers? Is D&D (and Pathifinder) so popular because it's fantasy? Or is fantasy so popular because D&D did that first? Or some other reason?
Sci-fi gaming is hard. It's not easy coming up with all the reasons it's harder, but when I ran Alternity sci-fi, I found it
much harder than D&D, even though at the time I knew the Alternity system better.
To this day, the only sci-fi RPG I've seen done really well is FATE, although Star Wars SAGA edition works fairly well too. FATE is incredibly rules-light, and their sci-fi setting involves creating a star system cooperatively (the entire group will be as familiar with it as the DM) and ... you don't go elsewhere (the PCs can't suddenly appear where the DM wasn't expecting them to go).
I think it's also easier to come up with a fantasy setting. There's standard Western Europe medieval civilizations, and the standard races (elves, dwarves, and humans at minimum), and even character "classes". Other settings have to be more flexible, but this creates problems in writing rules and then running a game.
1. Low-level fantasy limits transportation and communication. This limits the scope of what a new player or GM needs to be "on top of" for a given session. Compare that to a game of Vampire: The Masquerade where the GM has to grapple with an entire internet of information being ubiquitously available to the PCs. Or to Eclipse Phase where players can spontaneously decide to transport their consciousness across the entire solar system if the whim takes them.
This is a big one.
DMs in 3.x often complain that wizards can "break" this (can Teleport anywhere, can Scry, can use Sending). One thing I like about 4e is the DM still has a lot of control over these things (especially teleporting). You can't just teleport or plane shift (in non-combat terms) wherever you want. You have to go to a teleportation circle, which the DM can make into an encounter. (If you discover the BBEG's teleportation circle number and teleport into there - and frankly you probably won't without DM buy-in or at least letting them know this is an upcoming possibility - you'll be facing some really nasty opponents, possibly a seriously overleveled encounter.)
I'm happy to play other games, but this is one of the reasons I prefer GMing fantasy games. I tend to overthink the things in the modern and non-post-apocolyptic future games and it drives me crazy figuring out how things manage to be ungoogleable, avoid being caught on any surveillance cameras anywhere, not be caught by someone on a cell-phone, stay hidden from satellites, etc...
A related problem I have with GMing modern games is that its too easy for me to worry about how closely the things simulate reality... and its kind of awkward GMing things where the other players have lots of real world experience.
And finally, I haven't found many other games that are as easy to have both "dungeon crawls" and investigative play.
The player and DM aren't always on the same playing field in a modern setting. If a player knows guns more than the DM, or computers, or anything else that pops up in a game, there's going to be problems. Making matters worse, people become obsessed with realism in a modern or sci-fi game. I don't see a whole lot of complaints about how unrealistic taking gunshots is in Warhammer Fantasy, but as soon as you switch to d20 Modern, you find out that two members of the group, people living in a big city in
Canada, know a huge amount about modern guns.
