Aluminum isn't a horrible conductor, copper is just twice as efficient at transferring heat. OK, so maybe it is. But al radiators still win due to other advantages. You can build an al radiator with much wider tubes having more surface area than you can with copper, so the coolant moves faster and dissipates heat faster. That's more important than the material's ability to transfer heat.

On a somewhat related note, if you ever see CPU heatsinks with copper cores and al fins, and advertizing claiming that al fins dissipate heat faster than copper, it's a lie. That's a purely cost-saving move. An all-copper heatsink works much better.