The thermal resistance of copper is lower than aluminum as is its electrical resistance. This is the case with all precious metals. From lowest to best conductivity of heat and electricity; aluminum, copper, silver, gold, platinum. That is the scale of the precious industrial metals excluding high noble metal processes and alloys.

However Alum = cheaper than copper. People already pay 200-400 for a new heater. They dont want to make the price 400-600.