What I am asking for is a reason why people accept, without having read any of the often mentioned sources, that Fallout depicts a postnuclear alternate future which diverged from the real 50s. It's like everyone, who is interested into the game played it and accepts a kind of canon saying "Yeah, this is what people in the 50s might have expected or feared." But WHY do players thinks so? I guess noone or the smallest minority read 'The World of Tomorrow' from August Derleth, which is often cited as a source for the future image of the 50s.
Well, in America at least, this is just kind of a part of our culture, I suppose. That distinctive visual style wasn't apparent only in the form of movies and comic books, but also influenced everything from the designs of cars, to refrigerators and toaster ovens. Back in the 50's, over here, there was a kind of a movement where products were designed to kind of make you feel that "The World of Tomorrow" was already here. (And when you get right down to it, that's still a common phenomenon - if you buy a TV or Blue-Ray player, you're still buying something that's been designed to look like it came from "the future.") There's even a word for that particular stylistic movement (though at the moment, I can't think of what it is.)
In the US, you can still see a lot of this today. If you drive across country, you'll come across gas stations that look like spaceships, or flea markets that sell toaster ovens with lots of aerodynamic fins, etc. Or growing up, I had http://en.wikipedia.org/wiki/The_Jetsons cartoon, stuff like that.
In short, the visual styles that Fallout borrows comes from a distinct visual style that was at one time very popular (at least here in the states) and even remains an influence today. There's an instant connection when playing Fallout 3 (for us yankees, at least) because that stylization is embedded on our social consciousness. It's referring to a distinct and well-known (at least on a sub-conscious level) aspect of that time period.
Edit: Another way of putting it is kind of like how if you see a movie that takes place in the Middle Ages, or the 1920's, or the 1800's, you just kind of immediately make that connection. There's no one "source" to point to, to say "oh, that's how I know what the Middle Ages looked like," it's just a lot of factors. It's the same thing with the "World of Tomorrow" deal (again - for us in America, at least.) It's hard to explain much better, because it's like trying to answer something like "how can you immediately tell that Robin Hood takes place in England in the Middle Ages?"