Why underground transformer neutral point prohibited grounding

Neutral point is not grounded system is to consider the issue of personal safety, due to the more humid underground, electrical equipment is prone to leakage, neutral point is not grounded, the firewire leakage can not form a circuit with the ground, the human body contact will not cause electric shock. In general, the low-voltage side of the grid are neutral grounded, so many low-voltage equipment is for security reasons, from the grid to take power after the addition of an isolation transformer, isolation transformer secondary side is not grounded.