Musc about sums it up for me too.
Fantasy is what (many) people are used to, and they start to cry if the usual stuff isn't there. Dragons, damsels, fireballs, the same classes and spells etc.
I've seen games that are more sci-fi oriented, but it still amounts to a similar gameplay concept - fighter(soldier)/magic(chemical)/archer(grenader). And they usually - not always of course - don't sell as well. Thus I'd assume business wise it's about that aspect.
Consumer-wise, perhaps players tend to associate tech/guns/space with shooters/action/strategy and not RPG's out of long habit. And as we all know, some habits die hard.