Wodan
Deity
Well, because bronze working really was a prerequisite for iron working. Pretty much everybody had iron; it's common. Only a few people had tin, and you needed tin to make bronze. The tin trade was of huge importance during the bronze age. Despite the fact everybody had loads of iron, nobody was making iron tools for quite some time after bronze metallurgy had become ubiquitous. The reason was that to smelt and forge iron, you needed much higher temperatures. Ovens that could do this weren't developed right off the bat, they happened as a result of centuries of bronzeworking.
Iron itself wasn't superior, initially. Early iron implements were both soft and brittle compared to bronze (plus, they rusted). It took lots of refinement before iron became the superior material. The reason some people adopted iron was because they couldn't afford or were cut off from tin, so they couldn't make bronze anymore. It was a cheap substitute, at first. In Europe it appeared in the backwaters of the time - mostly up around Austria and in the 'barbarian' fringes north of Rome and Greece.
So actually, someone could have developed iron working first. Consider a planet without any tin deposits at all. Wouldn't they have to go straight to iron working?
It sounds to me as though Bronze is easier to work, and that's why people in history adopted it first. Yet, it is not a true prerequisite and there's no reason a civ could not have gone straight to Iron.
Part of the problem is doing a justification after the fact. Somebody somewhere mixed tin and copper and made this good alloy. Then everybody copied that guy. So it's convenient to presume that way is the only way things could have happened.