Nadal's game is best suited for the clay courts of the French Open where the balls are slower. In the grass courts of Wimbledon, the balls are faster. He is always in danger of being ousted in the first few rounds when the grass is still immaculate and very slippery. During the later rounds, specially around the baseline area where Nadal plays, the grass would already be trampled. It would not be as slippery as during the first few rounds. This is when Nadal's ability to track down balls come in.
Nadal is also still suffering from the knee injury. He hasn't yet fully recovered from it. He won the French Open for the eight time but not as convincingly as he did the seven other times.