As roids are illegal The Rock can't talk about them. And he might be a bit in the same position as Mentzer at the time. If dwayne states what he is using many insecure or desperate boys will follow his stack.
Without steroids The Rocks career would never have gotten anywhere. Steroids did everything for him. All the money, movies, wrestling. Without the extra 50 lbs of muscle, without the extra definition and masculinity he would have been a tall but unnoticed average fit looking nobody.