Why where Americans encouraged to grow their own food in victory gardens?

During World War II, the United States mobilized for a massive war effort. Many men from the agricultural sector were going off to war, meaning it would be difficult to do as much planting and harvesting. Further, trains and truck that had been used to transport food to grocery stores were being diverted into the war effort—and the gas that fueled trucks was being rationed. Finally, and most importantly, soldiers going overseas needed to be fed, and fed well, so that they would in the best possible form to fight. They received top priority during the war in terms of the US food supply. Food for average civilian American was therefore strictly rationed.
In order to avoid demoralizing food shortages, the US government strongly encouraged people who had any lawn space to plant vegetable and fruit gardens to supplement their diets. The government also offered classes in gardening and canning. More than 20 million people did plant victory gardens, so called as they were expected to be part of the push for an Allied victory. Beyond providing more food, the gardens helped people back home to feel a part of the war effort.
It is also important to note that food was a more pressing concern before World War II than now. It was not until after the war that scientists engineered the widespread use of chemical fertilizers, pesticides, and the genetic food engineering that significantly increased crop yields per acre and, therefore, world food resources. Hunger was a very real threat during times of war in the early 1940s, and the government responded robustly.