Nathan Fillion is wrong for the role. Sony want a blockbuster, and there's no way that Fillion's name sells tickets to the general public. No matter how the geek community feels, the guy just happens to be a solid TV actor, not a name that can be prominently displayed on the poster of a multimillion dollar blockbuster.
On the other hand, we have a certain Mr Bradley Cooper. He's young, he's good looking, he has the right charming attitude for the role, he's proved he has action chops by doing the A-Team, he's currently one of the biggest stars in Hollywood, and most importantly, he's worked with the director already.
Now I ask you. Who is the more logical choice for the studio and for Neil Burger? Because from where I'm standing, it's certainly not Nathan Fillion.