Subutai was a military genius who realized that long term, the life of his own troops was of enormous value. He took incredible care in his campaigns to minimize Mongol casualties. He often did this at the cost of time (which is a pretty darn smart way to do things). You can read more about this in history books. That engendered great loyalty among his troops. He was a master strategist who was able to analyze multiple data points and come to a plan for war that was almost always perfect. He was able to do this by combination of innate ability, deep knowledge of his own soldiers and of the enemy and an intense focus on gathering knowledge about his enemy (which was rare for that time) and also a bit of luck. He was one of the first generals to make heavy use of scouts and spies (for 100s of miles in each direction). This was extremely useful in the open lands where the Mongols often fought. He always knew more about his enemy and knew it earlier. Also above all, he had the toughest, meanest, most badass fighting force (almost entirely made of cavalry) fighting under his banner.