Because it’s cheaper and lighter than a copper wire of the same resistance per length; about 6 times cheaper and twice as light as copper.
Aluminum has 61% conductivity of copper, but only 30% of weight.
So, a bare aluminum wire of half as much weight as a bare copper wire will have the same resistance, which is important in power transmission lines that span about a few hundred meters between towers.
Even if the prices of aluminum and copper were the same, this would also mean that an aluminum cable would cost half as much.
Since aluminum is roughly about 3 times cheaper than copper, a bare aluminum cable would cost about 6 times less.
