Why were pennies changed from all copper to copper plated zinc?
Several issues are important here: (i) the cost of the metal; and (ii) the ease of machining the metal into rounds to make coins........
And I must admit, I don't know much about either issue. Presumably, the inherent value of the metal in the coins is not an issue. We could add a third issue: the durability and lifetime of a copper coin versus that of the zinc plate. Again I have no data.