Copper is expensive (3 times more expensive than aluminum)
Copper is heavy (3.3 times heavier than aluminum)
Copper is a good in conducting heat
But the problem is, copper only brings a ridiculously marginal benefit talking about heat dissipation!
For same level of weight and material cost, CM can bring you much much larger effective fin area for cooling down the temp; that's why the Hyper 212 Deluxe edition finally became a "Dream" edition, and only kept available in the dream but never realized in true world......