Why are there no good AF 50mm lenses available for Nikon?

I have no complaints about the Nikkor 50mm f/1.4G on my D700 and D800. It is quite a decent performer. Acoording to Photozone.de the Sigma 50mm f/1.4 is a good performer with high sharpness in the centre of the image, but it really falls behind the Nikkor in sharpness on the borders and in the edges.

Sigma test http://www.photozone.de/nikon_ff/616-sigma5014ff?start=1

Nikkor test http://www.photozone.de/nikon_ff/441-nikkor_afs_50_14_ff?start=1

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Jur