I don't know the actual reasons for which Hayden Christensen wasn't cast as Edward (personally I don't think he really looks the part anyway) But one reason that Robert Pattinson is good for the role of Edward is that he can act real classy, just like Edward.
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Hayden Christensen was not cast as Edward Cullen in the movie Twilight because the producers felt that he was too old for the role at the time. They were looking for a younger actor to portray the character. Ultimately, Robert Pattinson was chosen for the role.