The Mongols failed because the Khan died and the army had to retrain and after that the empire fractured. After that the 4 major part all deal with many local issues and also fight each other.
I really have to read up on the Mongols, and central asian history in general. That subject is terribly undercoverd in Western history. For somewhat understandable reasons, but still.
And I'd hardly say the Mongols failed just because they couldn't hold to western Europe.