Kevin,
Honestly, I think a lot of people like to see good, talented people fail because it makes them feel better about their own underwhelming or miserable lives. Manning is one of THE best in the NFL from a number of different perspectives. However, if he had six Super Bowl titles under his belt, 6,000 passing yards/year and 50 TDs/year, people would hate him even MORE.
Seeing someone like Manning fail somehow makes people think that their own "pedestrian" lives don't completely suck as much as they might have thought.....
Misery loves company.