By the time of Peloponnesian War (431 B.C.), the poeples of the Aegean and Asian Minor had posessed iron-working technology of sort for over 800 years. However, Iron rusts away faster if not taken care of, and was harder to shape into armor and hammer into desirable shapes -- mainly because for iron to soften enough to be worked by hammer, it requires a higher temperature, and often a different design for furnace with bellows. In this period, Iron tools and weapons existed, but they remained very expensive -- the average farmer would have only a one or two good iron knives that they would take very good care of. So they had iron, but continued to use bronze for many applications, for a variety of technological and economic reasons. Armor and many larger metal vessels (pots, pans, etc), as well as many luxury goods like mirrors and jewelry, continued to be made out of various types of bronze alloys.

Think of it this way: In the 21st century AD, we could make cars much lighter and more fuel efficient if we made engine block and body panels out of titanium instead of steel. But we don't. Why not? Because titanium is considerably more expensive and harder to work than steel, even if the titanium is stronger for it's weight, but it's also more brittle -- it will snap in two rather than yield to pressure and deform. So it's great for bicycle and aircraft parts, where weight is more of an issue, but less good for things like armored vehicles, where the ability of metals used in forming the frame and armor to deform under stress, instead of exhibiting brittle failure (cracking or splintering) is very important.