Tire rubber succumbs to dry rot for two basic reasons; lack of use and exposure to sunlight.
When tires are used regularly the compounds in today’s tires will be released and helps to keep the rubber soft and pliable. When you store tires, on or off the car, the resins and oils in the rubber are not distributed and the rubber oxidizes and breaks down. Flexing of the rubber distributes those compounds and the tire will last much longer.
For similar reasons, sunlight will do the same thing to tires. It will cause the resins and oils in the rubber compounds on and near the exposed surface to break down and/or evaporate… Storing tires for long periods of time is bad and storing them in sunlight for long periods is the worst case.