The US made pennies of copper (95%) in 1941 and 1942, and partially from old shell casings in 1944 and 1945 (so the copper content may have been slightly lower in those years). Only in 1943 were pennies made of steel (with a zinc coating) in an effort to save copper for use in the war effort.
Probably because they needed copper for the world war 2 effort.
ww2 the need for wiring in the military Short answer: The US was using all the coper for munitions used for WWII. copper was required for the war effort i.e. to make electrical cables