Wouldn't wearing metal armor be like wearing a lightning rod? It's certainly a better conductor than human flesh and there is a clear route from the armor to the ground. But obviously magical lightning doesn't behave like normal electricity else it would jump to a nearby target that can easily have a charge induced in it, say the metal amulet around the caster's neck, or a dagger lying on the ground nearby.
Let's say the magic somehow controls and projects the electrical discharge in order to put it onto the target. In that case, the metal the target is wearing is going to be a very effective defense, the opposite electrical charge to that produced by the wizard can be induced in the metal armor far more easily than in the target.
Even if the wizard's magic is inducing the opposite charge in the target, the metal still presents a likely decoy. It's like trying to throw a magnet at someone on the other side of a metal pole. Even if the magnet is enchanted to be attracted to the person, the pole still presents a viable decoy.
I'm not going to claim with scientific certainty that I know metal armor would protect one from lightning, but I would like to know on what scientific grounds people are claiming metal armor makes you more vulnerable to electricity.
But I don't think electrical attacks in DnD work anything like electricity. They work more like beams or missile attacks. We're complaining that a spell doesn't follow a (possibly incorrect) notion that we hold about electromagnetism, but we don't mind the fact that it breaks every other law of electromagnetism out there.
