There are articles in the ScreenCrush archive going back to 2014, where The Rock was “confirmed” to appear in the first Shazam. But by 2017, it was decided to give the character his own movie, instead of including him as the antagonist in Shazam! Speaking of that decision, Johnson spoke about it recently in an interview with Vanity Fair, where he revealed that it was very much his call and his desire to separate Black Adam from Shazam. Johnson said that when he got the early script for Shazam “it was a combination of Black Adam and Shazam: Two origin stories in one movie,” which he felt would “be doing Black Adam an incredible disservice.”
Dwayne Johnson has been attached to the character of Black Adam for a very long time.
Or maybe the balance between the innocent Shazam and the cynical and vengeful Black Adam could have made a nice contrast!
So Johnson himself lobbied to get his own Black Adam movie…