Interesting, Brolin doesn't go into great detail about it but he does mention he was in talks with Snyder and things just didn't work out.
“We did. I didn’t have conversations about it, but Zack [Snyder], that was part of his idea. It just didn’t turn out. It’s OK. It is OK. You know, another scenario might have worked better, but I’m happy for Ben.”
Brolin also showed his support for the casting of Ben Affleck and outrage over the negativity of his casting.
“I’ve never seen such a global reaction in my life. I feel for him, truly. I would not want to be him right now. The reaction becomes so personal. It’s like, ‘Fuck this guy, I wish he was dead.’ And you’re like, ‘What? Dude, seriously? This guy is just working like you are. He’s doing the same thing you are. He’s trying to make the best choices he can.’”