They didn't. Some fittings were changed to metric when the veicle was upgraded to EFi, like the front radius arm securing bolts and the metalistic bushes and bolts (the bushes are stiffer) but the back axle stayed imperial for some time.
Likewise the engine, the heads are different from the introduction of the EFi so some bolts were metric, but not all.
The axles themselves became metric with the introduction of the ABS style axles and then the 300TDi