What would cause primitive people to use metal?

I was wondering why some cultures around the world used metal for a very long time and others never did. Asian and European people used metal even at their poorest, but in powerful Latin-American cultures like the Aztecs, Mayans and Incans, they did not ever use it even at their peak. I wonder why? Also, there are some cultures ( Africa 100 years ago comes to mind) that had the ability and knowledge to use metal but scarcely used it. Why would they p up using it more frequently?