Nickel and chrome oxides are insulators anywhere near room temperature; copper oxide is a weird semiconductor at best.
What I think you mean is that the nickel-silver oxide film is comparatively thin, and self-healing, compared to the progressive scurf of oxides and sulfides and stuff that affects brass. So there is less 'insulator' in the layer, at a reliable thinness, for conductivity.
Sometimes we forget that chromium is a hellishly reactive metal -- it forms oxide in a heartbeat in contact with oxygen, and this layer is what effectively 'passivates' items that are chrome-plated. (Or that have chrome in the alloy). Nothing more is needed to keep the underlying metal bright ... and some of its valence electrons reasonably mobile...