I think it's indicative that the most popular sci-fi RPG presently on the market is Star Wars, which really isn't science-fiction, but science-fantasy.
And then there are RPG settings like Gamma World, which, again, are science-fantasy. (Beneficial mutations from radiation? There cannot possibly be any such thing! No, those are just another form of magic.) A true "post nuclear apocalypse" setting (such as that given in Twilight 2000) would be bleak and dismal for most RPG'ers, though for some it might be interesting and challenging.
And then there are RPG settings like Gamma World, which, again, are science-fantasy. (Beneficial mutations from radiation? There cannot possibly be any such thing! No, those are just another form of magic.) A true "post nuclear apocalypse" setting (such as that given in Twilight 2000) would be bleak and dismal for most RPG'ers, though for some it might be interesting and challenging.