As hinted at, but not elaborated on by the first part that mentions temperature monitoring, those wires in free air expand and contract with temperature changes. The cables need to be able to support their own weight without collapsing or touching other conductive objects like other cables or trees.

Adding more conductive material helps with the resistive losses, but also makes the cable heavier. Using an advanced cable that has better tensile strength for the same or better conductivity accounts for those second order effects better than just adding more aluminum.