Because he did an interview with Empire magazine where he said this:
"When you read the book," says [Robert] Pattinson, looking appropriately pallid and interesting even without makeup, "it's like, 'Edward Cullen was so beautiful I creamed myself.' I mean, every line is like that. He's the most ridiculous person who's so amazing at everything. I think a lot of actors tried to play that aspect. I just couldn't do that. And the more I read the script, the more I hated this guy, so that's how I played him, as a manic-depressive who hates himself. Plus, he's a 108-year-old virgin so he's obviously got some issues there."LOL.