The Witnesses are accustomed to having various levels of privilege and responsibility. Each with its own "school" so to speak. From the first publisher training book supposedly given only to those ready for baptism, "Organized to Accomplish Our Ministry" to the Missionary School at Patterson, the JWs are trained to accept levels and with those levels the appropriate training that is only available to those who attain that level. It is a given that all are accustomed to or are expected to accept. Not too dissimilar from other religions and organizations.
I've never heard an explanation for the secrecy, this too is a given. Just as the "Organized" book is not placed in field service or studied with someone who has not yet gained unbaptized publisher status. It is the way it is in the Watchtower. To be fair, though, I doubt that the rank and file Roman Catholic or Baptist or Mormon or Presbyterian, or whatever, has access to all the "privileged" material their priests or ministers may have access to or have learned in their "schools" because if they did many would leave the church.
Thus the shock and popularity of Bart Ehrman's books which open the door to what priests and ministers are learning at their seminaries and religious schools.