Aluminum has a lot of weird strict rules. It is cheaper, but the labor is more expensive. You can slap copper together quick and it will be safe and work perfectly fine for decades - there is no real world advantage to doing any better. For Al you need to take a lot more care to follow the rules or you have a fire in a few years.

Al is typically used for the large wires - the cost of the wires makes it worth the extra labor. For your typical house though the cost of labor is more than the wires, so Al is more expensive than copper.