The main reasons are simplification and camera-based control. On newer Nikon systems, aperture is typically designed to be set from the camera body rather than on the lens. Removing the aperture ring reduces parts, cost, and mechanical complexity, which can improve reliability.

It also avoids user error: on some older ring-equipped lenses, if the ring wasn’t set correctly, the camera could show an error or refuse certain modes. Eliminating the ring makes operation more consistent.

Another benefit is lens design and sealing. Without a rotating aperture ring near the mount, it’s easier to improve weather sealing around that area.

In practice, many Nikon bodies make body control quick anyway. Higher-end DSLRs often have a dedicated control wheel for aperture, and even many lower-end models let you change aperture directly in aperture-priority mode without pressing an extra button.

The downside is mainly for certain specialty uses, such as some manual, reversed-lens, or non-electronic extension setups, where a physical aperture ring can be convenient or necessary.