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.
Does the labor actually cost more? It's not that much extra work to use a torque driver and some anti-ox paste. If anything I'd expect buying aluminum rated terminations and outlets would be the biggest additional cost. Might be a little harder to pull aluminum with the larger diameter I suppose.
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.