Because he's got a hell of a lot of geek credits under his belt. From Firefly to ODST to Doctor Horrible's Sing-Along Blog, everything he does seems to be awesomely geeky. It doesn't hurt that he's a good actor.
Plus, like Chuck Norris, he radiates an intangible yet obvious field of awesomeness.
Plus, like Chuck Norris, he radiates an intangible yet obvious field of awesomeness.