I believe Apple were unhappy with the fallout of the 8600m GPU issues they had on the 2008-era MacBook Pros, due to poor quality solder. These same issues carried over to the 9600m GPUs on the 2009 line. Apple (I believe were the only OEM to admit to this as a fault; all overs denied it, including Nvidia. This led to Apple looking bad, and Apple isn't a fan of that. 2013 was the last generation to see an Nvidia GPU (some 2013 iMacs had a 750m), but they'd already begun the transition to AMD with the 2011 MacBook Pros with the 6xxx series.
Ironically, these GPUs also had serious flaws (although inexplicably, mine still works fine!) but it seems the damage was done between Apple and Nvidia, and they apparently (publicly) haven't looked back.
It's a shame really; ignoring the fact that the Mac Pro 2013 form-factor was a bit of an own goal, having a compact space meant that power was at a premium. Making use of cards that are more power efficient and generate less heat seems like an ideal fit for Nvidia. Alas..
Fingers crossed Apple does something a bit more standardised for the 7,1. You can't define a workstation as a commodity.