Because as has been shown with VSR vs. DSR and traditional Super-sampling analysis by ComputerBase, NV induces more blur and even has contrast issues. It's all there at Conputerbase. It's not in every game but there are enough examples that peoce AMD's super-sampling produces superior IQ in enough games for this not to be a fluke.
BTW, up to and including Kepler, NV still hasn't provided Full RGB support over HDMI. I haven't used Maxwell yet but I know Full RGB requires a fix for all GTX 200/400/500/600/700/800 NV cards or you will never have accurate true full colours over HDMI:
http://techreport.com/forums/viewtopic.php?f=3&t=90500
You and I both know most NV users are unaware of this. Again, chuck another major graphics issue to ignore / not important status because NV...