The fronts CJ faced last season were crazy. Plus the Titans OL is vastly overrated as a run blocking unit. I think some are sleeping on him this year providing Hasselback and Locker can form a more decent passing attack.
The problem with Madden though is to reflect a team's overall ability to rush the ball, the RB ratings have to suffer - it traditionally has done a pathetic job of representing proper run blocking abilities and scheme based advantages (like having a top passing attack to spread out the D, i.e. Foster).
Over the past two seasons, CJ has had more rushing yards, rushing touchdowns, YPC, basically the same receiving yardage and more receiving touchdowns than Peterson, while being the ultra obvious focus of defenses and without any backup to give him a rest worth a damn. I respect Peterson's ability at RB but I can't sensibly argue his is better than CJ. The ratings as they are are simply to reflect the team's rushing ability due to lacking game engine ability, and probably a touch of the typical assumptions because he failed to get two 2000y seasons in a row.