This isn't brain surgery, rocket science, or even algebra. It's simple division...
Card Hash Rate Wattage Cost $/Hash
R9 Fury 300 H/s 150W $260 $0.86
1070 370 H/s 150W $400 $1.08
Keep paying $1.08 per hash, I will pay $0.86.
Not only that but they say resale value is an issue.
If I can buy 1.65 R9s for the cost of 1 1070... I can buy more gpus which when I resell will still net me equal money... Because although the gpu won't be as valuable I am able to buy more of them for my money.
Not to mention they ROI faster because they are cheaper.