The decision to reveal Alden Ehrenreich’s casting was apparently already set for three months away, specifically, the official unveiling took place at Star Wars Celebration in 2016.
Studios have plans for how and when every new piece of information is released, and that includes major casting decisions.
Because Lucasfilm wanted to wait to tell the world, the actor couldn’t talk about it.
Although, the actor’s casting had been rumored for several weeks prior to that.