I don't think this is necessarily something to put on Peterson. Over the years, the Vikings have added more offensive weapons, notably a better receiving core and aquarterback. With Brett Favre there this year, the Vikings had a passing threat that was lacking with Tavaris Jackson, so Peterson wasn't relied on as much to run the offense (This is similar in San Diego with LT - his role is reduced now that Philip Rivers has become a better qb). I also think that defenses are more prepared for him than in previous years, when he was first breaking out on the scene.