It's known because the character of Superman is invincible to most people. How does an invincible man die? It's the mystique behind the title that drives the story in the minds of non-comic book folk who only know generalizations of comic book characters. The Death of Superman isn't my favorite, but I think it's an interesting progression for the character that would translate well on film if written in a certain way. Like I said though, it'd have to be really... really god damn good writing.