So, we all know that the generic dungeon fantasy genre just *works* for RPGs. There are many I find that simply don't. I've been trying to explore those that I have thought would never work, that being superheroes, western, Extremely Specific fantasy (Tekumel, Talislanta, most wacky homebrews) or science fantasy (Metamorphosis Alpha), as well as those that should work but kinda don't, like Post Apocalypse and comedy (Toon, Paranoia). When I say "work" I mean can sustain a prolonged campaign. Perhaps I should get out more, but I think it's weird you hardly hear about multi-year Arduin campaigns or even extended Mutants & Masterminds campaigns. It's always D&D.
So what's the problem?
Comedy: Comedy games are made for one shots, convention games, etc. They are the "party board game" of the RPG sphere. Paranoia is the most audacious of these, and while it can totally work, the game is designed around an end result - that being all of every PC's six clones dying in hilarious ways.
Extremely Specific Fantasy/Science Fantasy: These games follow two roads because of excessively required player buy-in: They are simply D&D with a few odds and ends here and there OR they are dead on arrival as the GM gets heavy in to lore and the players (one or all) simply don't, and pull all the wind out of the GM's sails. Many campaigns die due to a failure of player buy-in, it's just the most starkly prevalent here.
Post-Apocalypse: This game often devolves to D&D with guns/lasers and likely extremely wacky PC types. The novelty wears off when NPC types are as wacky as PC types, and therefore there is no norm to buck with their existence. Being the same as everyone else is boring, especially if you're intent is to be a unique snowflake. Then all you have is D&D with guns. And when you're reminded of another game when playing a game, you compare the two and in the end D&D wins out on that.
Superheroes: There's no character growth in the genre. It's a serial affair, so the universe usually resets between comics, sometimes between storyarchs. But there's no real development. Nobody wants to see Peter Parker grappling with old age or divorce or homelessness. He has hard times, but he's never broken. Of course this is considering the golden, silver, and bronze ages of comics, not iron or later (which I call trash. We're in the trash age). Players have to be able to buy in to this. The easiest way to buy in is have people play established characters everyone knows. A Bat Family game could be outstanding, whether or not you have someone play Batman or have him as an NPC or even out of commission Gotham Knights style. But the buy in still has to be there. That's a core problem with these genres and is most starkly apparent in Supers.
Western: Western has a different problem, that being the aim of the game and the media which is heavily visual in presentation. A western was a type of movie first. I mean yes there were penny dreadfuls and pulps but the genre exploded with the advent of motion pictures and even more with colorized motion pictures. Aces & Eights is pretty much the only game to try to incorporate a western's visual presentation, and therefore is the best western game by default.