Why were the Mongols able to conquer so much territory so quickly?

The Mongols grew up on the steppe where life was difficult and required endurance. They fought each other for many generations, until Genghis Khan united them and created a huge army. They rode tough horses, moved very fast, were expert horse archers, learned from the enemies they defeated and were extremely disciplined and mobile. If a Mongol warrior showed cowardice in battle, they killed not just that warrior, but his entire squad. Their families went with them during military campaigns and supported the army. The Mongols were a poor people in material terms until they began their conquests, so they had a big economic incentive to conquer. The Mongols developed good tactics, including the feigned retreat and psychological warfare, fought with bows, arrows, lances, swords, knives and siege weapons, and were extremely ruthless and violent. The Mongols were entirely cavalry, while their enemies often had infantry components and defended castles and walled cities. Their military success and credible stories of cruelty encouraged some to surrender before a major siege or battle. The Mongols were not out to change anyone's religion, culture or economic system as long as the subject peoples paid tribute or taxes. They moved rapidly to surprise enemies and lived off the land they invaded. Mongol raiding parties could often move at will in the enemy's territory, because of their speed and mobility. Genghis Khan had sons, grandsons and great generals who could attack in many different regions.