Why did the USA start to mint steel pennies instead of copper pennies in 1943?

The 1943 steel cent, also known as a steel war penny or steelie, was a variety of the U.S. one-cent coin which was struck in steel due to wartime shortages of copper. It used the same design that Victor David Brenner had made in 1909 for the copper Lincoln cent. Due to wartime needs of copper for use in ammunition and other military equipment during World War II, the United States Mint researched various ways to limit dependence and meet conservation goals on copper usage. After trying out several substitutes (ranging from other metals to plastics) to replace the then-standard bronze alloy, the one-cent coin was minted in zinc-coated steel. This alloy caused the new coins to be magnetic and 13% lighter. They were struck at all three mints: Philadelphia, Denver, and San Francisco. As with the bronze cents, coins from the latter two sites have respectively "D" and "S" mintmarks below the date.