Because Apple is being infantile and not using newer Nvidia drivers.
Not only are 970/980/750 not in the OS drivers, neither are the Titan Black and 780Ti, despite having been around for a long time.
It seems to me that throughout 10.9 they never added any new Nvidia cards to the shipping drivers.
If instead of dragging their feet they just let those newer drivers become part of their OS, this whole event would have passed unnoticed.
I realize that choosing which pink watch band goes with the "Aluminum Sport Extended" is high-tech stuff and needs most of their engineering staff, but maybe every now and then they couldld spend a few moments on actual computers.
Netkas to the rescue !!!
Yes, I can confirm that this works.
Nice thing with our EFI cards you can do this without removing cards, etc.