I just thought of another reason for using Al this way.
Since the heat is being transferred to air, which would be much slower than the 2, using larger fins with lesser thermal conductivity might be more desirable than smaller fins with higher conductivity.
This would also be accompanied with other design changes like greater fin-fin gap, which is better for use with lower pressure fans.
So overall cheaper design and lesser noise.

Of course one could still use Copper fins with the same design for marginal performance increase, at the cost of more weight.