I think the reason is quite clear.....money and convenience! Apple got a good discount deal from AMD so probably they have signed a long term deal of exclusivity. Something that benefit Apple and benefit AMD because they were struggling to stay alive. Also it makes life easier for apple, because the 5K screen requires some additional stuff which I dont understand, Im sure they prefer not making sure it works for all kinds of GPUS. So laziness and money I guess.
I don't think its quite as clear as that although I'm sure money is a HUGE factor. AMD had to collaborate with Apple for their proprietary single tile 5k P3 display TCON. We don't know much more than that, for all we know nVidia could have said to buzz off.
AMD and nVidia have different business models. Neither AMD nor Apple are currently struggling especially AMD with their contracts. I mean come on, you are 5x more likely to see/use an AMD product than nVidia in your everyday routines....Playstation and Xbox alone almost guarantee that.
What I dont understand is that Apple has been striving to be considered a premium manufacturer of computers, and you would expect them to pick out the bast hardware options for their customers to choose from. Especially in the high priced BTO options, you should be able to pick the very best components....heck they charge you out of the nose for it. Right now Im starting to feel like the donkeys in Tom & Jerry, sitting with a classy slick machine knowing I paid tons of money for, and for everything else than the lightweight tasks, the machine doesn't perform great. Apple seem to be more focused on the outside than the inside of these machines.
You just feel nVidia is more premium because they currently have a better GPU line out. And with the use of Vulkan those lines are being more and more blurred. The Fury X was AMD's version of the Titan X (previous generation) and it held its own.
This is reminiscent of saying the Galaxy S(whatever) is better then the iPhone simply because they have different launch dates. Of course the newest thing from either competing company is going to be better.
I never been extremely focused on the detailed specs of every component.
But, currently the gap between Macs and what you can actually get for the big bucks regarding hardware has become too big. I just ordered my own PC components the other day. the machine Im building will be something like 4X faster than my maxed out 5K imac in pretty much every aspect. and it will cost me about the same as it would to buy the next iMac 5K maxed out BTO. So, this is a better solution for me. Finally I will have a machine that renders quickly and with an awesome GPU - And I no longer needs to be told by Apple what GPU I should buy.
List your parts you are using....
Its literally impossible to build a computer that is 4x faster than a maxed out iMac at the same price. Even if you are doing something as weird as comparing it to last years iMac you still can't build a PC that is 4x faster at the same price. You'll exceed the entire price of the iMac in a quest finding a CPU that is 4x faster. The iMac uses m.2 PCIe storage, nothing that is 4x faster. Thunderbolt, USB, etc etc all similar. Memory can definitely really be improved but not 4x...maybe 2x. So really when you say "pretty much every aspect" you mean "only the GPU"?
Plus you are building a desktop tower not an AIO which the two shouldn't be compared so easily. And YOU are building it yourself if I build something equivalent to a maxed out iMac with a GTX1080 through another manufacturer I come up with this. (its hard to find everything Apple offers in a BTO pc especially a 6700k and m.2 storage but I found Alienware has something similar).
i7 6700k, 32gb RAM, 1tb M.2 SSD, Windows 10 Home 64bit, GTX1080 Founders, with an optical drive (I couldn't remove that so take off 50 bucks).
No monitor, no speakers, no mic, no camera, no thunderbolt. (I'm ASSUMING it comes with a mouse and keyboard although not positive about that).