Because the Slumdog Millionaire director Danny Boyle has picked him for the iBio-based movie which comes out on October 9. Mr. Fassbender might be a good actor but he doesn't look like the Californian hi-tech swami. At all. When I was watching the trailer I thought Boyle might as well have picked Arnold Schwarzenegger. Which was one of the choices on gizmodo.
I would personally have picked Noah Wyle -- he did a pretty good job playing Steve before -- or Keanu Reeves. An edgy actor like Bale would have been good too but he backed down. Daniel Day-Lewis would have been a great choice.