[HPsquared]

Aluminium is usually "good enough" relative to copper on both types of conductivity, and it soundly beats copper in terms of conductivity per unit weight (which is the metric of interest in many applications) and presumably in terms of material cost too. Copper is better if you want minimum physical size, though. (EDIT: Copper does have some durability advantages though which is another reason it's often favoured)

[AnIrishDuck]

It is indeed the conductor of choice at utility scale. It was even briefly used in place of copper wire in construction [1], though that usage was halted when safety issues emerged (differential expansion when mated with legacy copper pulled apart joints and created major fire hazards)

1. https://en.wikipedia.org/wiki/Aluminum_building_wiring

[IAmBroom]

I thought the problem was primarily galvanic activity between copper and aluminum, not thermal growth. A bit of searching says this is so in environments with any wetting or very high humidity. The increased resistivity of aluminum's oxide layer is also part of it.

Basically: they just don't play well together.

[AnIrishDuck]

Yeah that checks out too. I'm basically regurgitating what I was told when working the electronics aisles at a hardware store.

After reading the wiki page more thoroughly, there were just a bunch of different issues. Goes to show what happens when you make a seemingly benign but fundamental change in a system full of parts designed for something different.