There is a very steep curve involved in the smelting of iron. Unlike copper or tin or zinc (and their alloys such as bronze and brass), the melting point of iron is too high for most ancient furnace designs. And even if you manage to melt it, the product is an iron sludge, which because of the impurities, is actually worse than bronze. So it's an enormous amount of effort to produce an inferior product.

Even in the old world where iron was adopted, it was known for a very long time before its use became common. Although people knew about iron, they hardly ever made it because they already had a better and stronger material - bronze - which was also much easier to make. The only objects made of iron typically used meteoric iron, which was already pure, but such objects were scarce. As late as 1500 BC, the Hittites in Anatolia were using iron only for ceremonial purposes. It was too expensive and impractical for everyday use.

It was not until the 11th century BC that two crucial developments took place. The first was reheating the iron with charcoal to produce steel, and the second was rapid cooling (via quenching) to make it harder. But it also became more brittle, so quenching had to be combined with annealing before steel got to a point where it was an improvement over bronze.

This may be why iron did not become popular in some isolated parts of the world. The technology was too complex, too many steps had to be discovered before you had a useful product. It may also be that the pressure to develop iron technology was not there. There is some speculation that in the old world, the invasion of the sea peoples led to disruption of trade routes, and thus produced a scarcity of tin for making bronze. This may have forced people to develop iron.

Regardless of iron metallurgy, people all across the world have been using meteoric iron for a long time, which is ~95% pure and may not even require smelting, just cold hammering.

You might get more of a response from this in /r/AskAnthropology.

The creation of iron is rather complex and probably would not be apparent to a group that had no contact with iron or saw no need for it. For example, the Inca People(they were not called that, but just for clarity) did not have iron because they did not really need it. For the terrain and varied climate, armor made from extremely tightly woven rope could prevent errors from piercing them and allowed for flexibility and comfort. For weapons, various other metals were available and stone weapons could work as well. Iron just was not needed so it never appeared. Also, iron appeared first in central Asia so the idea of it travelling all over Europe, Asia, and Africa makes sense as they all had contact with one another.