This is a tedious read that is mostly tangents from the headline. The question of “why copper?” is mostly about why not aluminum, silver, or gold. The article touches on aluminum but never bothers to answer why not silver/gold.

This article has no central thesis and is instead a mishmash of random facts about electricity flow through circuit boards (often weirdly specific at that). Don't read this article hoping to find an answer to the question posed in the headline

Gold is not great for bulk traces, it has worse conductivity (thermal and electric) than copper. It's used for coatings because of its corrosion resistance and its incredible ductility.

Silver would be better than copper, but the difference is not large (~8%) enough to justify the price differential.