What military and economic reasons explain why the mongols were able to build an empire so quickly?

The mongols were a nomadic steppe people who were masters of horse warfare. To put this in perspective, the modern country of Mongolia is often claimed as the most sparsely populated country in the world. Genghis Khan united the mongolian tribes and did away with the traditional class system in his army. In addition, the Mongols took on many different ethnic groups absorbing many of the other nomadic peoples especially those of Central Asia into the fold. (skipping detail here) Their military tactics were ruthless, but effective. Using the standard techniques of nomadic raiders and then augmenting and improvising the mongols were never afraid to use genocide against groups who opposed them. They put so much fear (for good reason) into their enemies that the Mongol hordes feature prominently among the folklore of Asia, Eastern Europe, and the Indian subcontinent.