Why do we buy milk by the gallon and a bottle of coke by the liter?

In 70s we were not much familiar with coca cola. Pepsi were sold in liter and milk is traditionally sold locally sold in gallon.

Explanation:

Sarah Brew said that unlike to the global brand like coca cola, Pepsi are sold at global level but milk is sold at local level follow the local custom measurement. If we look back in history, the milkman sold milk door to door in the form of quartz and the gallons. Same things consumers expected from seller.

These are two ways that play a role in grocery shopping. That's  why the congress make a product list in form of liter and metric and traditional inches , gallons, pounds, milk etc.