In a way, I am not entirely happy about what I am discussing, but this might be because of my position on mechanics, and the ability to carry settings rather more easily than other things.
Settings sell, and the colour of games sell. The opposite also holds true, because we use the game’s setting and colour to decide if we are to buy the game, quite often. As I’ve told elsewhere, I was initially not sold on Tony Lower-Basch’s capes, because I shy away from superhero games, and it was billed as a superhero game. I purchased it once I’ve been convinced I could take from it what I wanted without dabbling in supers.
There is obviously the movement within story games to tie colour, or at least theme, to the mechanics, so you should be able to pick a setting and find mechanics that support it, or pick a set of mechanics and find a setting that encourages them and the kind of story you wish to tell. But you can see how much it is not so with how many hacks many games receive, and even if the games retain their theme, the colour (sci-fi as opposed to a WW2 story) can often be quite painlessly switched.
I wonder if a part of the issue comes from when many games were more or less identical, and the only thing to separate them, and even their themes, were settings. Even if the mechanics are the same, it’s not always the same kind of story. Heck, why go to “similar systems” when we can remain within the same one? Ravenloft and Forgotten Realms give rise, or enable different stories. Different World of Darkness games give rise to different themes, even if their mechanics are often similar enough (or let’s look at games of Technocracy and Traditions, Kindred of the East and Vampire: the Dark Ages if we’re going to be sticklers). Settings are what we had to differentiate games, so we chose by settings.
But I don’t think it’s that simple. While the metaplot in the oWoD was quite good at polarizing the player-base, if you liked it, then there was nothing like it to suck you in. It sucked me in. Even if the system was not always to my liking, I’d really enjoy thinking of the world that had been created.
And if the system was not to my liking, it’s not like I couldn’t just replace the system. For you see, there wasn’t a place as much as in the oWoD where people told me they played the game while without blinking an eye told me they didn’t roll dice, at all. For the setting is considered the game.
If you’re not a system-geek, and even if you are, settings and colour are really important at whether you’ll buy a game or not. This is the allure of licensed settings, this is why generic systems create several setting worlds.
It works. It sells.