Nadal and Federer have been the number 1 and 2 for the last six years. Why exactly?

From a technical standpoint, I believe their forehand technique allows them to hit at a higher degree of plane variations than the loop swing that is used by Djokovic, Murray, Davydenko and others.

I also think their footwork and stance they employ are far more flexible than the others. This allows them to hit a wider range of shots from their ability to improvise their steps.

Your observations may not be so technical and might be owing to mental toughness or focus. But I would like to hear what your reasons might be.