Because Nadal has consistently struggled on fast courts, besides the few exceptions. Also, it's simple fact that the courts have slowed down considerably. It's his biggest help but also his biggest enemy, he needs the slower speeds to thrive yet they damage his knees.
I like the guy but you gotta see that at least.
Grass is a better surface for Rafa than slow HC as seen by his performances at WI compared to AO/IW/Miami.
Your statement is incorrect.