It's being trickled out to manage reactions for whatever engagement metrics they are trying to go for.
If they released the casting first there would be an overwhelming backlash and the movie would be written off easier, with a stronger narrative of failure behind it.
It had gathered a narrative of failure anyway, but not as strong or intense as it would have been if the casting was fully announced when it was completed. Delaying controversial casting announcements mitigates the intensity. People who were favorable to the movie before the casting announcement will still give it a chance.