The Mongol invasions of Japan failed because Typhoon winds destroyed the Mongol fleet. The Mongols failed to conquer Japan even though they had successfully conquered the much larger land of China and also Korea. The typhoons that saved Japan did not even happen just once but twice ! Both times that the Mongols set out to conquer Japan, their fleets were destroyed by typhoons. Japan was geographically in luck and also helped by nature.
Why did the Mongol invasions of Japan fail? The Japanese were superior fighters. An outbreak of smallpox had weakened Mongol troops. Typhoon winds destroyed the Mongol fleet. The Mongols could not make enough weapons.