Iron ore is readily available. Metallic iron is not readily available in nature. The technological issues are that higher temperature are required for the smelting of iron (i.e., converting ore to metal) than for copper, tin, etc. In addition, iron doesn't offer any great mechanical advantage over bronze (eventually, when steel, and especially hardened steel, was developed, the much higher hardness was an important advantage, but (low-carbon) iron is about the same hardness as bronze), so until it became common enough to be cheaper than bronze, there wasn't much motivation to use it.

An earlier discussion where I wrote about the technology and /u/Antiquarianism wrote about the social aspects: https://www.reddit.com/r/AskHistorians/comments/7ryt2v/why_was_there_a_widespread_bronze_age_before_the/