The overhead lines that come off the power pole and connect to your house or business are Aluminum, and the higher voltage lines that the poles are holding up could be Aluminum too. But if they're older then they could be cloth covered solid copper.
I actually have some solid cloth covered copper wire that the power company tossed in the town dumpster a few weeks ago when we had power poles snap.
But as for the reason they use Aluminum is it's cheaper to purchase, and is lighter to work with. And it's less likely to get stolen as although it still has some scrap value, it's not worth as much as copper.