The main technical reason is flange focal distance—the space between the lens mount and the sensor. Nikon F-mount lenses were designed for SLR cameras, which need extra room for the mirror mechanism. Nikon 1 cameras are mirrorless, so the sensor can sit much closer to the lens mount.

That shorter mount distance is a key advantage of mirrorless design, but it also means an F-mount lens cannot sit directly at the correct distance from the sensor on a Nikon 1 body. An adapter is needed to add back the missing space so the lens can focus properly.

So the new mount wasn’t arbitrary: it reflects the different physical design of a mirrorless camera. Using the old F-mount directly would give up one of the main size and design benefits of the Nikon 1 system.