Back in the early 2000's, There were folks speculating that Johnson would be in the movie. Nothing official had been announced, but some folks said that he would make a better Black Adam than a Captain Marvel. MTV Blogs held a poll on the matter. The results were resoundingly in favor of Black Adam, and Johnson said that he would be happy to talk about playing the role. As the years dragged on, the project fizzled. Then, about a year or two ago, Johns started dropping hints that he would be in a superhero movie. There was much speculation as to who he would play. Ultimately (and, to those who had been paying attention and knew what they were talking about), he announced that he would be playing Black Adam.
There is not even a script completed yet (as far as I know), and knowing what the story is and how the character is going to be handled will have an influence on the casting decision for the World's Mightiest Mortal (whether they call him Captain Marvel or Shazam). They are not going to cast such an important character before they know what the character is supposed to be.