Was it because Djokovic reached a level so high that peak Nadal could not match? I say peak Nadal because he was playing at a similar level to 2010 which is his best ever season. Was it purely a mental issue with Nadal given his victories over Djokovic in 2012 as proof? I'm not sure what to make of it. re watched the Rome final in 2011 and have come to the conclusion that Djokovic was just to good and if both were at their peaks then Djokovic would win most of the time. Problem is that I don't think Djokovic has ever reached that level he was at from Jan-June in 2011. 2015-2016 is so overrated compared to his 2011 level because in 2011 the tour was ripe and there wasn't a lost generation of players like we have now.
Something to think about, losses to players other than Djokovic in 2010:
Roger Federer [SUI] d. Nadal
Jurgen Melzer [AUT] d. Nadal
Guillermo Garcia Lopez [ESP] d. Nadal
Marcos Baghdatis [CYP] d. Nadal
Andy Murray [GBR] d. Nadal
Feliciano Lopez [ESP] d. Nadal
Andy Roddick [USA] d. Nadal
Ivan Ljubicic [CRO] d. Nadal
Andy Murray [GBR] d. Nadal
Nikolay Davydenko [RUS] d. Nadal
Fed got him at the WTF. Murray knocked him out of the AO and Canada.
2011:
Roger Federer [SUI] d. Nadal
Jo Wilfried Tsonga [FRA] d. Nadal
Florian Mayer [GER] d. Nadal
Andy Murray [GBR] d. Nadal
Mardy Fish [USA] d. Nadal
Ivan Dodig [CRO] d. Nadal
Jo Wilfried Tsonga [FRA] d. Nadal
David Ferrer [ESP] d. Nadal
Nikolay Davydenko [RUS] d. Nadal
I'd say for sure that the elephant in the room was 2011 Djokovic.
But I would not conclude from this that peak Novak is better than peak Nadal. For me that's a bridge too far.