Because it would make Zeiss look bad. Dx0 has been waiting until Zeiss Milvus line release so they, Zeiss will have a lens for Sigma to knock around which is closer to Art series cost. Did you notice that 50mm was the only Art that was not reviewed on Nikon D810? 24mm and 35mm has been but not 50mm - why? Because it would make 55mm Ottus (4K lens) look bad (Otus is an amazing lens but 4K?)
Otus on 5d MK III scored 38 with sharpness of 21 P-Mpix
Otus on D810 scored 48 with sharpness of 33 P-Mpix
35mm Sigma Art on 5d MK III scored 34 with 19 P-Mpix
35mm Sigma Art on D8100 scored 42 with 30 P-Mpix (funny how zeiss has +10 over 5D MK III but 35mm Sigma Art has only +8 - based on the sharpness being +12 vs +11 for sigma I say it will be +9 for D810 for Sigma 50mm Art)
So, based on the above - if:
Sigma 50mm Art on 5D MK III scored 35 with sharpness of 21 P-Mpix
Sigma 50mm Art on D810 should score 44 with sharpness of at least 32 P-Mpix
What is even more peculiar is the fact that at the same sharpness on 5D MK III 21 M-Mpix (sigma 50mm Art vs Otus 55mm) Otus scores 3 points higher while CA is the same for both, Vignetting is .3 EV lower on Sigma as is distortion (by .1%). Only transition is .2 EV better on Zeiss but somehow Zeiss scores 3 points higher? (please dx0 - if you say that Vignetting and distortion differences are minimal REMEMBER that so is transmission difference!!!) - I guess at least 2 points are given to Zeis because of the name?!? :) - I like Dx0 - I love Optics Pro 10 but PLEASE, try to be a bit less obvios in your Carl Zeiss butt kissing, please????
<div id="linkdxomark">This a comment for <a href="http://www.dxomark.com/Cameras/Nikon/D750">this page on the website</a></div>Dpreview
"Sigma's latest 50mm is one of the very best lenses we've ever tested" in fact its the top lens on a nikon. why did you test it on a canon but not on a nikon ?