The easy answer is because the resulting alloy of tin and copper was more durable and useful than copper alone, which had been in use for at least 1,000 years before the first production of bronze, about 3,000 B.C. But, nearly as important as the discovery of the art of alloying was the requirement that large groups of people had to work together to secure the ores, build the furnaces, acquire the fuel (or make charcoal), feed the workers and many other allied chores to support the industry. It's little wonder an entire epoch or age was so named after the metal...