I found a great article about the differences and commonalities between the science-fiction, fantasy, steampunk and post-apocalyptic genre: Postapocalyptia and Genre Bending in Nerd Fiction
"Post apocalyptic worlds give you an environment where it feels totally organic that elements of both sci fi and fantasy would exist coherently side by side." – Greg Simon
I guess this is one of the reasons why I love the post-apocalyptic world of Fallout. I never got to like the fantasy genre of videogames because I always look for an immersive gaming experience where I can dive into the story and put myself into the shoes of the protagonist. I found it very difficult to get this experience from fantasy or sci-fi games whereas the remains of a destroyed Washington D.C. always seemed more pleasant and appealing to me.