Why is The Curse of Strahd appealing? 

Because player-characters aren’t really fighting the vampire Strahd, they’re fighting Dracula.

Why does the Forgotten Realms remain one of the most popular campaign settings?

Because the players are really running their characters  late-medieval Europe sans Christianity, Middle Earth, or whatever popular fantasy novel series they’ve read. Heck, maybe they’ve starting playing D&D because they read the Drizzt novels.

The same goes with the Old World in Warhammer Fantasy Battles (well, before Games Workshop blew it up).

The players aren’t pushing armies in the Old World, they’ve chosen sides in a darker, pseudo-medieval Europe with monsters.

The science-fiction games out there are in the multitudes, but each try to recreate the experience of space opera like Star Wars, Star Trek, or Dune. What’s the balance between “hard” and “soft” science? Is there some sort of “Federation” or “Republic” to police the galaxy?

Cliché works. Cliché is welcome. Cliché is familiar enough to most gamers to be marketable. 

And if they haven’t been exposed to it then it’s not cliché.

 

What really matters is execution.