Reasons for Nadal's AO Results:
1. Poor Preparation.
Nadal thrives on match wins, and developing confidence that way going into major tournaments. He never gets that with the AO. His leadup to AO has always been unusual imo, not only starting a new year with very little tournament play but no match wins on the continent. It's fine for someone like a Djokovic, but I don't think it is an ideal preparation for Nadal, who needs the confidence boost of match wins. Thankfully this year his team (maybe Moya?) had the idea to change his pre-AO schedule and play a leadup tourney on the continent and it helped him quite a bit.
2. Level of Competition
The level of competition to Nadal has always been fierce at AO. One of his biggest rivals, Djokovic, plays his best at this tournament. A big barrier that has stopped Nadal from winning at least one extra AO.
3. Shocking Luck / Injury
He has had everything from a bad case of the squirts (blame Qatar), viruses and also his back going out on him (and Wawrinka getting HOT) to blame for not doing better. For sure injury affected him in the 2010 & 2011 versions of the event.