3) Genre conventions: I find it hard to believe how rigid some peoples mind is about what is "right" or "wrong" in a certain genre... insert the smallest dose of steampunk into your cookie cutter tolkien fantasy, and some people will grasp for their torches and pitchforks.
I was told once "mixing fantasy and present day / science fiction doesn't work, there is no fanbase for that" when suggesting that for a game. That certainly is not true, though the fanbase of orcs in space certainly is smaller than the fanbase of fantasy orcs in loinclothes or cookie cutter aliens in space. But the point is that there are people evangelizing the "you cannot do that, like, ever!" sentiment, and being quite vocal about it.
Warhammer 40k is the most well known game (available in many forms) that takes Orks into space. It seems to have it's dedicated fanbase.
Its kind of a trope -- same as "space marines". We tend to like and make these things because they're established enough to be familiar, yet they're not closed systems so we can change and expand without alienating people. Fantasy also spans a wide berth though, so probably seems bigger than what's objectively Tolkien-esque -- for example, I wouldn't say that the Final Fantasy or Dragon Warrior franchises have much in common with Tolkien's world, even if they took some inspiration from it; but they do very much fall under the heading of Fantasy.
I do agree though that there are other worlds and mythos and real cultures that are largely untapped. There are relatively few games that tap Norse mythology, for instance, or (especially outside of Japan) the Japanese feudal period or themes of, say, Bushido; almost no games have explored African themes, Native American themes, Central American themes, or South American themes. Very few games have explored a more-or-less contemporary setting for game styles most-closely associated with fantasy or historic settings (Earthbound is a notable example). American West themes are pretty untapped (check out Boot Hill Heroes).
There's inspiration everywhere and its a shame games haven't branched out more.
I think there are some games that use Norse mythology? I can't say I remember very well which ones. But also, the new God of War is going to be Norse.
Certainly there is a lot of possibility from African, Native American, Central American, South American, and Asian (South and East) themes but I think the major reason we don't see very many of those style of games is because most devs aren't too familiar themselves with those themes. Japanese devs are fairly familiar with Asian stuff, so I've seen some fairly common themes there as well.
There's definitely untapped potential out there...
I'd love to play some kind of a Bethesda-style Wild West RPG, if they weren't busy rehashing Tamriel and Fallout.
Or a Carribean pirate RPG that's not Pirates! Gold or the remake.
One game that I can't believe hasn't been made (well) is a Roman gladiator simulator/management game. First-person Mount&Blade style combat for arena matches mixed with something like a business simulator for running a ludus.
A good remake of Darklands...
A Bronze Age RPG modeled on the Odyssey or the Aeneid.
Monty Python and the Holy Grail - The Game. Help, help, I'm being repressed!
For the Wild West one, Red Dead Redemption comes to mind? I believe it's an RPG? Maybe I'm wrong. There was also Assassin's Creed Black Flag, tho it's not a true RPG either.
In general, fantasy RPGs I think are getting more popular for two reasons. One is that people are really familiar with the setting (devs and players alike) and that we see more fantasy in popular media these days too (think Game of Thrones, etc.).