A key part of the answer is the higher temperatures required for smelting (1200-1300C to smelt iron, vs 1100-1200C to smelt copper). These temperature ranges mean that copper smelting, but not iron smelting, can be discovered accidentally in earthenware pottery kilns (when using copper ores for glazing). This points to two possible routes to the discovery of iron smelting: accidental discovery when making higher temperature forms of pottery (e.g., vitrified pottery such as stoneware and porcelain), or accidental discovery during copper smelting. We don't know which of these was the historical path, but these are the most likely.

Iron smelting in primitive smelters takes place in the solid state - the smelting temperature is lower than the melting point of iron (unlike copper, where the melting point is lower than the smelting temperature, and liquid copper is produced). The typical product of the primitive smelter is a bloom, a spongy mass of iron and slag, and one needs to learn how to get rid of the slag (hammer, fold, weld, hammer, fold, weld, and so on until the iron is clean enough), which requires working the iron at welding heat (compare bronze forging, which is done cold, at room temperature).

Noting that where metallic iron (either meteoric iron or telluric iron) was available, iron-working could, and often did, predate iron-smelting, we can ask whether hot-forging and welding can pre-date iron smelting. Typically, meteoric and telluric iron is cold-forged. This restricts its use to small objects (arrowheads, small blades, beads). We know of no definite examples of hot-forging and welding of iron that predate iron-making. The iron dagger from the tomb of Tutankhamun appears to be hot-forged and forge-welded, but post-dates iron-smelting in Anatolia (but predates iron smelting in Egypt).

In addition, iron doesn't offer any mechanical advantage over bronze. Iron can be much cheaper, since iron ore is very abundant, but iron only becomes cheaper than bronze when enough is made. There isn't necessarily much motive to translate an accidental discovery of iron-smelting into an iron industry. (If there is pre-existing use of meteoric iron, iron-smelting might be a very attractive discovery - a synthetic source of the magic black metal from the sky.)

More discussion in https://old.reddit.com/r/AskHistorians/comments/aexgpk/if_iron_is_really_better_than_bronze_in_every_way/ and by me and u/Antiquarianism in https://www.reddit.com/r/AskHistorians/comments/7ryt2v/why_was_there_a_widespread_bronze_age_before_the/