It is mostly the way they are treated. VW Eurovans suffered the same fate...(we had lots of them around in Canada as a commercial spec diesel van).
I know of some well taken care of Sprinters, and they are mint/perfect. Normal washing and waxing/touch ups goes a long way to keeping them rust free. Peculiar is where the Sprinter rusts, the newer generation along the body trim in the middle, it must vibrate or hold a ton of salt as they would not be dented or scratched there. But in general just unloved and driven till they die that is why they are rusty.