AMD hardware at a specific price point is more powerful than nVidia hardware at the same price point. However, nVidia has superior drivers that eliminates the difference.

And since Apple prefers to use their own drivers, nVidia loses their main point of differentiation.

But of course the "Apple" drivers for video cards are basically vendor drivers with Apple doing QA & release management. But the driver is nVidia's secret sauce, they're not going to show it to Apple, since Apple is now a very competitive GPU manufacturer. At some point Apple will probably put their own GPU's inside Macs, so nVidia doesn't want to give them a head start.

AMD cares less about giving Apple a head start because they care more about the short term than the long and competing against nVidia.