Because the Nikon converter missed a solid lens system, and the results are great. Of course, a converter is always a compromise, but with the quality of a Nikon TC takes on the very high level instead. With the above two objectives can be in any case get results just fine.
By the way: At DX lenses, the Nikon converter can according to the list "Applicable Lenses" not be used! Unfortunately, not even to the FX-ready 70-300 (the yes on the D300 already without the help of a whopping 450mm TC yields). I would terribly like to also extended in this way.
PS. Just now I see that in the "Applicable Lenses" for the 105s is that is not possible with the TC of the autofocus operation. But with me it goes completely without hesitation. I have the new AF-S VR Micro 2.8 / 105th Strange, but because it's going well, I hate to continue to scrutinize the.