Let's start at the beginning. Smith is actually the one who developed the initial story. It took place several years before the original film and there were still pockets of survivors and the story centered around Smith interacting, bonding, and ultimately failing to save them. For months this was the story. Warner's didn't much care for it and Smith and Weiss eventually came on board and all parties agreed to change things up. Well this change is pretty insane, the film is no longer a prequel, it's a fucking sequel!