A man inside an insulated metallic cage does not receive shock when the cage is highly charged because the whole charge reside on outer surface of the cage.
If the cage was a non-conductive cage, then the current cannot pass through the cage so it cannot affect the person in the cage.
For example- If you are seated in a car (metal body) and the car is hit with lightning, it is very unlikely that the person in the car would be hit with the lightning.
