What is the reason why the neutral is not used on high resistance grounding?

The question doesn't provide enough detail to give a definitive answer. The neutral PD is often used in protective functions on high resistance grounded generators (when grounded through a transformer).

High resistance grounding limits the use of Transformers - you must be using balanced loads, since very little current will flow (typically around 5-20amperes max) to the neutral. Because of this, phase to neutral values (such as voltage) are meaningless. Any current flowing in the neutral will cause an offset in all three phases from neutral. Since the neutral is high resistance grounded, it does not take a significant amount of current to cause significant offset.