Originally Morrison considered to do only 6 issues, and ended up expanding the story to more:
"It was a little bit of that. I also felt like I'd been brought in to do the early Superman, and as I said before, I was only going to do six issues. But the story grew out, and once I added a few villains, that added some doors to other stuff that could happen. Once I'd done that, I felt that I should at least hint at what happened to Superman over those five years of his early life. "
https://www.cbr.com/grant-morrison-s...s-of-superman/