Tin was hard to come by in those days and necessary for bronze. Late bronze age led to civilizational collapse and trade networks that distributed tin broke down. Iron was more abundant and could be more useful by itself than copper alone. May have been used for farm implements at first.

The early processes for smelting iron resulted in a little steel which would be easy to collect and save for one big work. Over thousands of years you learn more and more of how to 'encourage this'

1. Once you know how to make a hot enough furnace, iron is cheaper to make than bronze. Instead of finding copper and tin ore separately, you just need red rocks and a whole lot of trees. Quantity can trump quality.

2. There was steel in use already prior to 1000BC. High Carbon steel was being produced in both Europe and Asia by 400 BC. When people say "the iron age" it includes the usage of steel (which is mostly iron).

Anemia? Also, the earth made way more sense when it was flat.

Transportation costs used to be huge.