The underlying issue is that the DCEU has the Rock signed up and eager to play a DC character, that the Rock is a legitimate box office draw at a time in which relatively few celebrities actually can drive box office on the strength of their names, but for some reason the DCEU has pencilled him in to play a villain and not a hero and then got cold feet about having him in the Shazam origin story.
Now, Black Adam has more recently been more of an anti-hero than a pure villain, and as I said above the Rock is enough of a legit movie star that maybe he can escape the curse of solo villain movies just not working conceptually or structurally.