Why are Moscow Mules served in copper mugs?

I really enjoy inspired Daily Doodle topics. A friend showed me a brewery cans cocktails, more specifically Moscow Mules, but I'm not sure the cans were copper. Then I wondered, is copper a necessity to a Moscow Mule, or what is a copper mug just something people did?

One version of the story goes that in 1941, Moscow Mules were invented by John Martin (who had purchased the floundering Smirnoff Vodka distillery in the 1930’s), Jack Morgan (who was the owner of Cock ‘n’ Bull and he was trying to introduce America to his own brand of ginger beer) and Sophie Berezinski, an immigrated from Russia with 2,000 solid copper mugs made by her and her father.

The two men were already good friends when they met at the Cock 'n' Bull. As they lamented their lackluster sales with their vodka and ginger beer, in walked Sophie with her solid copper mugs.

As Sophie would tell the story, the three of them spent hours to develop a drink that would bring together the fizzy nature of the ginger beer, the punch of the vodka, and the cold properties of copper to create the next great cocktail. There were a number of taste tests and a few failed concoctions before they stumbled upon a recipe for a cocktail that would solve all their problems.

The Moscow Mule was born on that day in 1941. The perfect combination of vodka and ginger beer, housed in a solid copper mug that insulated the drink, keeping it cold and enhancing its flavor and aroma.

There is, however, the disputes over the actual inventor of the drink include: Wall Street Journal in 2007 penned the Cock ‘n’ Bull’s head bartender Wes Price also laid claim to the actual developing of the recipe, one version skips over Sophie and claims Morgan's girlfriend had a copper factory, and one story even mentions a 1948 article that talks about the drink being invented on the East Coast.

Whatever the story, the resulting cocktail was one America would fawn over and was advertised by Hollywood stars while decades later, still being served in a copper mug, both for tradition, and to keep the icy drink cold.