Why hail forms only in cumulonimbus clouds?

When hail is formed, it is not instanteous nor fall immediately. Storms have a peculiarosity of up and down flowing winds. When a droplet of water in the clouds began to fall, the very low temperatures present cause the droplet so form a skin of ice. This droplet then goes thru a series of upward and downward journeys until enough ice is formed to the weight that it can escape the forming arena and thus falls to earth. It is well known, for instance, within each piece of hail, no matter the size, the original droplet still exists.