What was the main reason for the Mongols' success?

I would argue that there were two main resons for the
success of the Mongols in creating and maintaining their
empire.

Their success in creating the empire came from
their military prowess.  The Mongol military included both heavy and light cavalry whose
abilities and tactics were beyond those of the people who the Mongols
conquered.

But conquering the empire was not the whole
story.  The Mongols were also good at maintaining it.  This happened because they
strictly enforced laws that actually helped the people of their empire.  They made trade
much more easy and managed to suppress things like tribal warfare that had made people's
lives hard.  They allowed people to practice their own religions instead of imposing a
religion on anyone.  They even had a fairly flexible system of local governments that
could adapt to local conditions.

So, the Mongols succeeded
by having a stronger military, but also by doing things that improved the lives of the
people they ruled.