There had to be a "wait and see" approach from Gunn/DC/WB because even though Gunn is confident in his 'Superman' and it appears it will do alright at the box office, there had to be some hesitancy to all of it. If 'Superman' turns out to be a bad movie or flops at the box office, you can't really go anywhere from there. It's risky enough that 'Supergirl' is being made at the same time and 'Lanterns' is in development, etc.
Imagine what would happen to those projects if this movie was bad? There were a lot of moving parts here. It's not like Feige devising a plan using characters that had never been on the big screen before. If you miss with 'Iron Man' oh well, the rest likely don't get made. If you miss with 'Superman' it hurts the entire studio moving forward.