Why can shelf-stable foods be stored on the shelf at room temperature?

There are numerous techniques that make some foods shelf stable. The primary technique is to lower the water content of the food (some foods, like flour, are naturally low in water). Bacteria need water to grow and if there isn't enough water present, then the bacteria will not grow.

Foods can also be acidified. (See Acidification.) Or, food can be heated to ultra-high temperatures, so that it becomes sterile. Some irradiation treatments work in this manner. Once the food is made sterile, however, it has to be hermetically sealed (airtight). If not, the food can become recontaminated and pathogens and other bacteria can grow quickly.