Because aluminum radiators are only more efficient in terms of size and space when using high pressure fluid, for instance automobiles.That's awesome... aluminum radiators are so much more efficient at heat transfer (and cheaper to make) than copper ones I'm not sure why the PC insists on copper unlike virtually every industry (automotive, appliances, etc.)
Learn how to plan what cooling system to put in your car with the choices of an Aluminum Radiator vs. a Copper Radiator in Car Craft Magazine.
In PCs, where the cooling comes from air and not fluid pressure, the superior thermal conductivity of copper can be used to full effect.
Nice compilation of common misconceptions - Greenman100 (Tim Elmore) Read More
"Myth: Aluminum absorbs/dissipates heat faster than copper.
Reality: All thermal properties of copper are better than
aluminum. Aluminum’s advantage is that it is lighter and easier to machine. So,
if one were given a pound of copper and a pound of aluminum, you
might make a better performing heatsink with aluminum, as it might have more surface area to dissipate heat than copper for a given weight. Considering footprint limitations for air cooled CPU heatsinks, however, copper is definitely favored."