Why do you think it is forbidden for anyone but a priest to visit the Dead Places?

Because a priest is the only person who can administer extreme unction and other holy rites

If you are referring to the "By the Waters of Babylon" why priests are the only ones allowed to enter the passages of the dead places. Firstly in many texts, priests are said to have magical powers and have elected rites to the titles. Secondly, they bless the dead and have the capacity, in that they can move through these areas because of their "charms" and "spells". These magical powers they possess allows them to purify and conserve, and secure themselves after delving into these spheres. Maintaining balance of the human and the necro side.