Why is the penny the only copper coin?

Answer 1

Actually three US Coins have been struck in copper, and modern US cents are mostly zinc with a thin coating of copper.

The use of copper dates back to the early days of coinage when coins had to contain their face value in metal. I.e. a dime contained 10¢ worth of silver, a half dollar had 50¢, an eagle had $10 worth of gold, and so on.

Copper was the only metal that was (a) inexpensive enough and (b) soft enough to be made into low value coins. Also, at that time there were also half-cent coins so pennies weren't the only copper coin.

By the 1850s inflation made the half-cent coins almost useless so the denomination was discontinued. The size of the cent was reduced to its current diameter and the composition was changed, eventually being made of bronze.

A few years after that the Mint experimented with a 2 cent coin, which was the third denomination made mostly from copper.

Since 1964 coins haven't been required to be worth their metal value so the composition has been changed to cheaper metals, but for a number of reasons the sizes and colors of most US coins have been kept the same as they were a century and a half ago.

Answer 2

The penny is the only copper coin because the cost of producing a copper coin became higher than its face value. The U.S. Mint changed the composition of the penny from mostly copper to primarily zinc with a thin copper plating to reduce production costs.