Ok, I know I'm going to get shot to pieces for saying this but.....
The brake disc is just a piece of bare metal, as soon as it gets damp it will corrode. Leave the van parked a few days in the rain and there will be a build up of rust. On a van the brakes are designed to stop it fully laden with a tonne or so but most of the time our vans don't carry this weight so the rear brakes don't do any work, the front discs quickly polish as they do all the work, the rears don't and gradually they become pitted. It happens on cars too but to a lesser extent as they don't have to cope with such a variation in loading.
I struggle to keep the rear discs polished on my van as it is not used every day, if they have to be replaced so be it. Badly scored discs will (or should) fail an MOT.
Tin hat on