it's technically like four different reasons.

The big one is weight per foot of current carrying capacity. this reduces the amount of transmission towers you need, reducing the amount of labor, and raw materials. the aluminum itself is cheaper, but that's a secondary effect. Since it's not as heavy per unit, you can make the runs much cheaper. And since you can make them cheaper, you can carry more current by simply making them bigger, or adding more of them.

compounding effects are something funky.