Ultimately just because the earlier axle frequently breaks; while the later one almost never does (under normal driving anyway, the racers still break them).
The later axle uses tapered roller bearings to carry the side load in corners, which are much stronger than the simple ball bearings used in the early axles. The connection between the shaft and hub was changed to a locking taper that locks tighter over time (making them very hard to separate) instead of straight splines that can wear and work loose. And I believe (not certain) that the half shaft is thicker through the important "overhang" area between the bearing and hub.
Haven't done it myself, but from what I hear, the swap is pretty easy. You'll need the complete assembly including brakes (probably best with the smallest slave cylinders); which then just bolts up to the springs, hydraulic line, and handbrake cable.
I'm sure you know this, but for any lurkers who might not : The rear axle assemblies were of course not actually made by Girling or Lockheed; the names refer to the kind of brakes that were installed at the same time. The early cars had Lockheed drum brakes all round; the axle assembly changed at the same time as they changed to Girling brakes with front discs.