Because you can't simply separate the mediums, which portray the characters.
Actually, uh, yeah, you can. Blade
, for example, is a wholly different take on the material than its comic source, and yet they're both wholly valid interpretations -- just as last year's Transformers
wasn't hurt in any way by Optimus Prime being a Peterbilt truck instead a cab-over.
Just because The Dark Knight
didn't translate, beat-for-beat, your favorite comic stories and origins, doesn't make it any less of a movie. Try criticizing the movie based on what it was