I think Irsay hinted as to why they were parting. During the news conference he talked about rebuilding the team, which to me would be a hint that Manning would not be happy playing with a bunch of newbies who are learning from scratch.
Manning probably agreed that he would not be happy, because even if he played 100%, when you have an otherwise all new team, the loses pile up too quickly.
Manning probably agreed that he would not be happy, because even if he played 100%, when you have an otherwise all new team, the loses pile up too quickly.