My guess is the backside of the rockers catch debris/moisture and it forms there. GT/P's just get it covered with the ground effects, but SE's and Century's get the eye sore-ness of it.
Regals show the same fate, but have the mouldings on the bottoms of the doors and rockers to cover it up.
x2 on the strut towers, check in the trunk and see. 97-00 W-bodies had a different design for the wheel well liner compared to the 01+'s. 01+ cars had a much better design, preventing the rust from forming on the rear towers.
