One of the difficulties it a horse winning the 3 races is that they are close together and each one is longer then the previous. Empire Maker did not run in the Preakness and he was well rested. For a horse the quest for a Triple Crown is similar to a marathon race. That's why it's so difficult.
The Triple Crown is one of the most difficult things to accomplish in the world of sports, obviously. PatrickM: Funny Cide won't be peforming stud service. He's a gelding.