Hi there. Apologies for the delay on this.
This mod generally sets aluminum as the metal that is used earlier for basic electrical items, whereas copper enters the game for electronic items. In the case of many things, like electric poles, this represents real life more accurately as copper is almost never used in such cases. In the case of AAI small electric motors, it's less clear. Copper tends to be more common in real life, however, aluminum gets a fair share of usage as well. So to keep the early game progression clearer (and it is already more complex), I chose to go with aluminum as the only option.