Why is the metal sacred to the people in the story

Metal represents technology and something they do not understand. It is sacred because it has properties that the tribe cannot produce:

The north and the west and the south are good hunting ground, but it is forbidden to go east. It is forbidden to go to any of the Dead Places except to search for metal […] These are the rules and the laws; they are well made. It is forbidden to cross the great river and look upon the place that was the Place of the Gods—this is most strictly forbidden. We do not even say its name though we know its name.