The assumption in the question is that Iron weapons and armor are somehow better than Bronze, and I base this on the fact that the Bronze age predates the Iron age.
Copper and Tin seem much more scarce, especially considering that Tin was mainly only available from the British Isles. Why is it that this alloy of metals, one of which was very difficult to find, was used by ancient people when Iron was much more plentiful?
Although iron is plentiful it has a much higher melting point (1538°C) than copper (1085°C) and tin (just 231.9°C). To get your furnace hot enough to melt iron you need a pretty snazzy furnace by bronze age standards. Copper, meanwhile, can be smelted at temperatures that you could coax out of a fairly ordinary pottery kiln without any special modifications or design features.
Iron is also a lot harder to work than bronze. A bronze ingot can (and in some cases must) be cold-worked without applying heat to the metal while working it. Iron on the other hand can be quite brittle when it's cold and requires a source of heat and a little more technical knowhow to work properly.
In short, Iron requires much more sophisticated metalworking technology to use and so for a long time iron was mostly used decoratively or on a small scale (jewelery, beads, status weapons, ornaments) until people got better at working with it. Then as more people figured out the techniques and equipment required to use it the way they used bronze they started using it more and more.