How come the 70mm F2.8 DG MACRO | Art is not available in Nikon mount?

The combination of focus mechanism (i.e. an extending, floating focus mechanism) with the focus-by-wire system used in the SIGMA 70mm F2.8 DG MACRO Art presents certain concerns over affinity with the Nikon AF-P (the “P” refers to pulse motor, also known as stepping motor) that affects user experience. For this reason, we are putting the development of a Nikon mount version on hold.

* Issues with user experience?
The lens remains extended and would not go back into place when the camera is turned off, which means you will need to cover the lens with your hand and push it back into place manually before turning the camera off. With other mounts, the lens will go back into place automatically when you turn off the camera.