It was the middle of The War and they were running short of copper, that's why.
UPDATE: The 1943 Copper Penny Is Worth $40,000 Or More
From the US Mint:
40 1943 copper-alloy cents are known to remain in existence. Coin experts speculate that they were struck by accident when copper-alloy 1-cent blanks remained in the press hopper when production began on the new steel pennies.
A 1943 copper cent was first offered for sale in 1958, bringing more than $40,000. A subsequent piece sold for $10,000 at an ANA convention in 1981. The highest amount paid for a 1943 copper cent was $82,500 in 1996.
Because of its collector value, the 1943 copper cent has been counterfeited by coating steel cents with copper or by altering the dates of 1945, 1948, and 1949 pennies.
The easiest way to determine if a 1943 cent is made of steel, and not copper, is to use a magnet. If it sticks to the magnet, it is not copper. If it does not stick, the coin might be of copper and should be authenticated by an expert.