You should ask why the Mongolian forces were so strong in 13th century?

I think there are five reasons:
1. Mongolia is a nomadic group. They can adjust themselves into very hard living conditions. They were good at riding horse and archery.
2. The great hero Genghis Khan was born. He made the Mongolians unified together.
3. The nobles of Nuzhen people bullied Mongolians, which made the Mongolians have awareness of suffering.
4. The productivity of Mongolia was weak. Every person hoped to seize wealth through wars.
5. Before the Mongolian society changed to be a feudal society, slavery was practiced. The slave owners needed a large number of people to serve them.