Neil Gaiman's Law of Superhero Films:

…the closer the film is to the look and feel of what people like about the comic, the more successful it is (which is something that Warners tends singularly to miss, and Marvel tends singularly to get right)

…Which, I suspect, is why Sin City and 300 worked. They were like having the comics happening up on the screen. The thing that people liked about it was there. With The Spirit, what the reader responded to is Eisner's lightness of touch and mastery of story, his humour and his humanity -- and a world that looks like Eisner drew it. The moment that it's obvious that that isn't there it almost doesn't matter what is there instead. According to Gaiman's Law, the more Sin City looked and felt like what people like about Frank Miller's work on Sin City, the more successful it was going to be with audiences, but the more The Spirit feels like Sin City and not like Will Eisner's The Spirit, the less successful it's going to be.