I'd suppose the fashion goes somewhat with the price of copper, though I haven't tracked it. The heatsinks themselves have gotten far larger as CPUs and GPUs have gotten more power hungry, not to mention RAM and SSDs. A material that's a good tradeoff at one scale isn't necessarily one at a different scale.

At any rate, one should expect many of these trades to go the way of Al if Cu gets more expensive (which it might not). Not all of them, but we'll probably see a bias towards physically larger systems in cases where space isn't at a premium. And also a bias towards active systems over passive, liquid cooling over air, and so on.