Because it is so hard to make, nobody used iron before about 1500 BC . Then the Hittites in Western Asia did learn how to use it. But they quickly saw that iron weapons were better than bronze ones, and so they decided not to tell anybody else how to make iron. The Hittites kept the secret of making iron for about 400 years, but when the Dark Ages came to Western Asia, the Hittite empire fell apart anyway, and the secret of making iron got out to other people. From about 1000 BC to the present day, iron has been the most used metal.
Steel is iron with more carbon in it than regular iron.
People first began making steel under the Roman Empire and the Parthian Empire , but really the first good steel was made by the Arabs , around 1000 AD in Damascus, and it was called Damascus steel.