[Edi]

As was already said, lightning resistance is much more difficult to get than fire resistance. Besides, want cheap fire resistance, why not use Dragon Helmet? It gives 100% fire resistance, or it should, unless it has been downgraded to partial only, which can make it an attractive choice to free up a miscellaneous or hand slot for something more useful.

If copper plate only gave partial SR, it'd be almost completely worthless.

Edi