Because he made a large base of Juve fans adore him to a point that they consider him a club legend no matter what and would "understand" his move to a rival club.
I never liked Moggi, but I consider him a club legend for what he did to Juve from 1994-2006.
being guilty of calciopoli or not, that is another story.
Edit: I wouldn't care if he joined Milan or not, in fact iirc, media was calling it a done deal for Moggi, Zlatan, and Buffon before Calciopoli appeared.
What I asked is why now he comes and say that he was going to Milan??!!