For a studio to green-light any project, they have to be fairly confident of a substantial return at the box office; even the greatest film ever made is a colossal failure if it doesn't at least recuperate its budget.
That is precisely why we tend to see the same comic book and superhero films, reinterpreted and repackaged, over and over again. If it's worked before - surely it will work again... It's a flawed logic, of course, because exceptions like The Amazing Spider-Man ruin the model, but you can sort of understand the thinking.
With the marker of success increasingly creeping towards the billion dollar mark, Hollywood mostly refuses to push the boat out into unknown territory for fear of alienating their mainstream demographic. And inevitably, darker or more controversial plot lines from the comics are shunned to avoid cutting off the lucrative family audience.
Given this trepidation, it should come as no surprise that the backlots of Hollywood are strewn with the corpses of some incredibly interesting comic book movies that could have been made, as well as some iconic storylines from the comics that have somehow never made it to a full pitch. When it all boils down, it's simply because nobody in Hollywood has had the balls to do it.