It is likely forbidden for no other reason than the society has slipped back into believing in ancient taboos of the things that they are ignorant of. The reader is given no explanation as to why only priests are allowed to go there, but the reader is given a sense that the people have gotten used to simply having an unquestioning obedience of those rules.
The mention of the metal has always intrigued me. I often think that somehow the metal was radioactive and that is why people are forbidden to go near it/touch it/etc. Long ago touching the metal could have killed a priest or somebody, and then the rule was made and never questioned. It's a possibility, but again there isn't much in the actual text that alerts the reader to any big single reason of "why" it is forbidden to go to the dead places.