Things change fast in the movie making world. What seemed like a good idea 2 years ago may not seem like a good idea 1 year later.
Maybe a few years ago, it seemed like a good idea to just replace Superman and Batman and keep everyone else but now the pendulum has swung towards full reboot.
Even over at Marvel, they had to get Iron Man right first. They released TIH the same year but it under performed. Then they took their time to make the rest of the movies.
Gunn has to make sure Superman: Legacy works. As long as Legacy works they can build the rest of the universe on that foundation. If he messes up the foundation, then forget building any cinematic universe.