Yes, there are many factors that need to be considered in product design. The performance of the material, the strength of the material, the cost to form the material, and the cost to manufacture the product. In this case, aluminum not only transfers heat better than brass, it's cheaper. And yes, an aluminum radiator will be designed differently than a brass radiator, so the total amount of material used, will be different.

And supercuda....the water and coolant used will be the same in both a brass or aluminum radiator so that's not even relevant for comparison purposes.