Legal aspects of modding.

At least, armor and monster ports from The Witcher to Skyrim are being published there normally, and there were also some from Kingdom Come, but I'm not sure.

The developers officially allowed ports from Kingdom Come for everyone. They also allowed ports from The Witcher, but only within the Witcher series. So, the rest is due to oversight. In general, there's a trend on Nexus towards a lack of, or at least a small number of, ports. For comparison, you can look at Steam, where there are a lot of ports.