Answer: The war cut off access to certain resources.
Explanation: World War II had a major impact on trade and shipping. The German war effort included trying to weaken its opponents, especially Great Britain, by cutting off supplies of food and other essential goods. German U-boats (submarines) attacked ships bringing commodities into Britain.
When the United States entered the war, rationing of many goods was introduced, in order to make the most of limited supplies and give full support to the war effort. Factories were very productive in America during the war, but factory production was shifted to things needed for the war effort itself. So consumer products became less available. Food items like sugar, butter, meat and more were rationed, as were needed products like gasoline and tires. (Japan conquered major rubber-producing countries in Southeast Asia, which cut off over 90% of America's rubber supplies at that time.)
The Soviet Union also used rationing after Germany's invasion of its territory in 1941. For instance, they set specific grams of bread for each person per day in the city of Leningrad, which was subjected to a prolonged siege by the German armies.
