Do we really need a reason? I'm just happy that he's not going to be in the film! I don't really care why he's not in it at this point. Christoph Waltz now has the part, who in my opinion is a much better actor. But, if for some reason you want to know what happened with Cage, he did a little interview with The Canadian Press in which he says:
I wasn't interested in just being just a straight-up bad guy who was killing people willy-nilly.
The Green Hornet' was something that I wanted to do, I think Michel Gondry is very talented and I had hoped it would work but I think Seth Rogen and Michel had a different take on the character.
I had to have some humanity and to try to give it something where you could understand why the character was the way he was but I don't think there was enough time to develop it.
What does killing people willy-nilly even mean!? It sounds like Cage just couldn't figure out the character and who he is. Maybe he's just a heartless killer that is just pure evil. Or maybe the character was just developed badly. I guess we'll find out when the movie eventually comes out.