Inverse-Proportionality; the more something is popular with the main-stream audience, the less it is with the core demographic. I mean outside of cheesy 90's fantasy shows like Hercules, Xena and Sinbad, main-stream always preferred Sci-Fi. We have what, GoT right now that is popular and that's about it? Almost everything else is Sci-Fi (even an average show like the Expanse got a second season and was picked up by Netflix).
People in general which tend to be part of the 'core' demographic prefer S&S settings since it's about fighting giant monsters, saving princesses and being an all around barbarian that can channel the fire of mages. They like the idea of simplicity under old tropes, the most complicated thing you will have in those genres is which prince will marry who and when will war break out before uniting for a common evil and you becoming king to rule them all.
Sci-Fi doesn't have any many restrictive settings as people claim. I mean Mass-Effect pretty much showed how to marry the idea of 'space magic', even Crysis had that element, with reality and you can still fight large monsters. I guess people in the core demographic just don't like the color blue and space-ships, or fighting Sci-Fi Dragons since princesses don't exist. Plus social politics, dystopia and question of human evolution are the main themes. We all know too well how Mass-Effect in the later games handled all that (let alone the newer Deus Ex series) so it's not surprising when it comes to games, the more Sci-Fi streamlined games are what work.
Same with War stuff. War games will always sell like hotcakes, the best we got recently was the White Castle series and even then it wasn't super popular. Main-stream demographic simply aren't interested in modding, core demographic will always be the ones to fire up a game 4 years later and mod it to play around.
Also, Fallout is Apocalyptic genre, hell, Apocalypse genre is the best when it comes to modding, since society both fills the roles of a barbaric world as well civilized progression. It makes sense that slutty outfits and even chain-mail would exist in a world like that, best of all worlds, but for some reason, it's in the mushy middle, where outside a few zombie main-stream things (ei; Walking Dead) and such it hasn't garnered much of a demographic base, Fallout did have a strong base mind you, but it petered out really quickly, even when compared to Oblivion or Skyrim, let alone Dark Souls in under a month.
Simply put, people prefer the simpler tropes of the genre, that's it, it doesn't make sense to have a Samus outfit in Skyrim, but I guess it does for them.