Bronze is an alloy of copper and tin. Weapons and armor crafted from this alloy have been the standard on Terratus for centuries. Only the most skilled smiths can reliably blend the correct ratio of copper, tin, and other materials to create a bronze alloy durable enough to withstand the rigors of combat.
Bronze weapons are lighter than the heavy iron produced by Kyros' Forge-Bound mage-smiths. And, while bronze weapons can grow dull in prolonged battle, they do not shatter like iron weapons. A dull blade is better than one that splinters in your face!
Though copper can be found in many mines in the Tiers, the only reliable source of tin was from northern merchant caravans who traveled to the Bastard City. After Kyros' armies conquered that trading hub, the Younger Realms of the Tiers were left without the ability to forge new weapons and armor.
The trade requirements and the skill required to create a durable bronze alloy meant that only the wealthiest of Houses and nations could afford to fully armor their soldiers in bronze. This put their soldiers at a distinct disadvantage when faced with the fully iron-clad legions of the Disfavored.