Good question. I think it may be just the way these cars are built that turns those mounts into regular wear items. They are common for needing replacement on the gas cars also, not just the diesels. The ones that wear out the fastest always seem to be the driver's side engine mount, since it is the one that gets pulled on when the engine is producing torque. The transmission mounts seem to suffer also if they get soaked from an oil or fuel leak.
At any rate, the motor mounts in these old Volvos have been known for frequent failure for a long time so I would imagine it's not an issue of low quality parts and more of an effect of the basic design itself. They are part of the Volvo safety concept, where the engine and transmission are intended to break free in an accident and slide back underneath the car rather than into the passenger cabin. The soft rubber and shear style mounting needed to achieve that seems to have the side effect of a short service lifespan for the parts. At least the mounts are not expensive or hard to replace.
