reasons its not done:
1 abs works by comparing the wheel speeds of the front and rear wheel, if one is rotating slower- then its slipping. so in order to fit abs to just the rear wheel would require all the expensive equipment that it would need for both. So its ecomonicaly a not good idea.
2. If the rear locks up its not a huge problem and can be controlled easily, if the front locks up your tasting bitumen!
3. No one reaches for the rear brake in an emergency which is when your most likely to lock up your wheel (eg this morning when a range rover tank pulled across me in the wet
)