Danyel<Snipped quote by chukklehed>
Technically it is a conductor in parallel with a human body. And when resistance on the conductor is zero compared to the human body, it'll go through there instead of the human body.
The insulator part is there to ensure it goes through the conductor, and the metal isn't directly into touch with the human body neither... armors do have PADDING, which has AIR, an insulator.
The body isn't hermetically sealed in the padding. There are plenty of places in the armor where it directly contacts the skin. I'm not saying that the majority of the energy might not pass through, but when you're talking about a bolt of lightning, even a fraction of it's power can be severely damaging. The steel can only conduct so much electricity before it starts to flow over.