Spirit-schmirit. He knocked over the right people early on and formed alliances with opportunists, same as any other warlord. Had himself a winning general too, which never hurts. I think Subotai was only defeated once while laying siege somewhere in China.
The empire was geographically big but because much of it was so inhospitable, its population was sparse, mostly nomadic, and ununified. Not exactly the stiffest resistance. Areas of urban development, like in China, slowed down expansion and took a lot more effort to topple.
Their military machine was efficient enough to make it all the way to Vienna, but their bureaucracy and succession planning was hopeless. The army withdrew from Vienna on the eve of attack and marched thousands of miles back because Temujin Ogedei died and no one knew who was in charge or what was going on next. Basically, Temujin Temujin and kid established authority over a lot of thinly populated land through military conquest and diplomacy, then the whole thing fractured and disappeared in a puff of smoke along with them.
It was an 'empire' like Alexander's was an empire - shortlived, built on conquest only, and relegated to infighting as soon as Big Chief died. It had none of the continuity, political development, contemporary cultural influence, or historical significance that you'd get with something like the Roman Republic/Empire.
