Comic book adaptations are a tricky trade. They obviously design money for movie studios,as evidenced by the four or so different franchises we've had running at any given time over the past decade, but they aren’t easy things to adapt. Writers and directors have to weed through long-standing, and often decades-long,backstory that is frequently muddled by the constant universe-altering events which buy place fairly regularly at the huge two publishers. They have to build a tale that is not only accessible to the layperson but also pays plenty of service to the hardcore fans.
Source: blastr.com