Bronze is an alloy of tin and copper and is lower in the reactivity series than iron. This means Bronze is easier to get out of its ore than iron. So bronze was found first and once we got good at getting bronze it was easier for us to extract iron.
Iron is a common element in the earth’s crust, but it almost always occurs as a compound, it is very rarely to find it naturally as metallic iron. Occasionally an iron meteorite will fall from space, to provide a ‘magical’ source of the metal. Such rare occurrences allowed a few smiths to discover the valuable properties of iron.
It is much more difficult to smelt iron than copper. In copper smelting, the copper flows as a liquid to the bottom of the furnace. The slag of waste material accumulates on top of the copper, and can be poured off to leave behind the copper. The copper is easily identified as copper, and the process is intuitive. But the physical chemistry of smelting iron from oxide or sulfide ores differs from that of copper, which may well have made iron smelting difficult to recognize even when it had happened. In principle, iron can be smelted from magnetite or hematite, which are comparatively common ores, but iron does not melt at the temperatures that can be reached in a primitive furnace: iron is still solid when copper and bronze are molten.
Answer 2: Copper is probably the reason why the Bronze Age came first. Copper is the perfect gateway from stone tools to metallic tools for primitive societies because:
Copper exists in nature in relatively pure form, and it’s common enough that it certainly would have been found in many places. Also pure copper is a striking attractive red-gold mineral.
Copper can be melted at wood burning temperatures, so it can be easily cast
Copper can be beaten into shapes with minimal or no heating
Copper tools are superior to stone tools for many types of uses.
The biggest problem with copper is it’s soft and doesn’t work well for tools that need a sharp edge: Axes, hatchets, knives, swords.
So the transition from the Stone Age to the Bronze Age in most societies actually was: Stone Age => Copper Age (short) => Bronze Age.
Bronze is an alloy of either Copper and Arsenic, or Copper and Tin. Once it was discovered that you could extract copper from its ore by heating it in a charcoal-fueled clay furnace, it was almost inevitable that bronze would be discovered next. That’s because copper ore often has arsenic in it, so copper smiths eventually realized that copper that was highly contaminated with arsenic was much harder than pure copper and was a good material for edged tools. (Arsenic/Copper) Bronze was predominate early on (Tin/Copper) Bronze became more common later, probably because arsenic is so toxic and would have been very hazardous for Bronze Smiths.