Because 4 reasons:
1. Longitudinal ribs instead of transverse. Fan will work with all heatsink.
2. 15mm (18%) wider.
3. Aluminum is able to radiate the heat into the air better than copper because of its lower density.
4. More distance between the ribs. Better for low air flow.
1. The direction of fins doesn't matter if the fan is mounted on top of it. It may make a small difference in case temperatures, but I think longitudinal fins do not cool the GPU any better than traverse fins. If it does matter though, that's still not a reason to go with aluminium, since a copper heatsink can also be made with longitudinal fins.
2. Yes, the heatsink is 18% bigger, but you have 47 fins instead of 74, so you actually have less surface area. Also, the farther away from the heatsink, the less effective the heatsink is. This is especially true for worse thermal conductors. By changing from copper to aluminium, the heat spreads much worse and the heatsink will dissipate less heat.
3. Thermal radiation is negligible when it comes to active coolers. Thermal conduction is much more important than radiation.
4. With the kind of temperatures you were getting in open air, you probably shouldn't design the cooler for low airflow scenarios, since you'll need quite a bit of airflow to properly cool this card.
With all that said though, it will be interesting to see how much worse the aluminium heatsink performs.
In order to improve performance, you should switch back to copper and look into a fan shroud to make sure the air actually goes into the heatsink, rather than around it.
You could also look into heat pipes. These will spread the heat much better across the entire heatsink, greatly improving the efficiency of the ends of the heatsink.
But design of heatsink (fin-to-fin air gap, fin length etc) is very important. My current copper heatsink have very small fin spacing. It perform very good with high rpm and very bad with low rpm.
