There were several things at play there. And I think Del Piero may have been somewhat responsible too. If you look at post 1998 Del Piero you have to be honest and say there were always fitness concerns. It's no coincidence that at his in fittest 2002-03 and 2007-08 he was marvellous. Especially in 2008 considering his age it was incredible. But there have also been numerous seasons when he showed glimpses of great skill, but condemned the team to a lot of hard work. Personally I think Del Piero did not look fit in 2011-12. I think he could have gotten in shape again and added another 2 years to his career at Juventus, but perhaps the motivation wasn't there or maybe it really was over. Either way he was just not fit enough IMO.
EDIT:
https://www.youtube.com/watch?v=1h8anjPHDsM
You know, this video is why I always liked Italian football so much. Look at the fans.. Some of them are not just crying, but bawling their eyes out. The sheer passion.. And the stadium speaker is so fucking eloquent. My God.