1. Copper is a good conductor of electricity but we use aluminium as a conductor in transmission lines .The main reason of not using copper is the cost .conductivity of copper is higher than the aluminium .But the cost of copper is much more than the aluminium .so due the cost factor we are using aluminum instead of using copper.you can check the conductivity of cu and al in the pic.
2 Another reason for not using cu is it easily gets oxidized.
We use aluminum i transmission line because it is cheaper than copper and the weight is lighter hence reduce weight on the tower loading.
Aluminum conductor steel rainforced (ACSR) is used for transmission lines. Aluminum has not very good conductivity and tensile strength like copper material but by aluminum rainforcedment, it’s tensile strength can be increased and as it’s cost is very low than copper material as well as it has low specific gravity, so this ACSR conductor is used in transmission lines.