Bronze is actually better than iron, its a harder metal. But it's pretty expensive since it involves the combination of two metals (copper and tin) usually found quite far apart from each other, while iron is just one metal, easier to find, and more plentiful. Therefore actually these early Romans were using the best and most expensive material to protect their head, and the cheaper material for the rest of their armoury. As the army expanded and they needed more armour for more people, and when the military stopped being made up of the patrician class, as the plebs joined in great numbers after Marius' reforms in 107BC, the use of the expensive bronze gave way to more cheaply produced iron.