I don't know that they rust any worse that anything else. We're up in the rust belt and just about anything will rust eventually. In my opinion it's a number of issues.
1. Some colors are quiet poor, like silvers and light blues seemed to just fall off, Ford, GM or Chrysler didn't seem to matter in the late 80's or early 90's. .
2. Care of the vehicle. If it didn't get washed and paint taken care of it will look like crap in short order. Rock chips are a real good rust starter, like the front edge of the hood and the front edge of the cab.
3. Some vehicles just have areas that are susceptable to rust, you can slow it down but it'll probably rust if exposed to the elements long enough. Remember the rocker panels on '61 to '71 Dodge trucks, what a design those were. My '91 cab is rotted bad above the windsheild, I have a few other '81 and up cabs that are bad also, most don't seem to rust but some do.
Just my comments and opinions,
Wayne