Nexus will not host mods that has content from other games. If any are uploaded, they will be pulled down pretty quick. It's done to protect the site and not have lawyers and DMCAs all over their backs. Using content from other games is a grey area and can be considered illegal. Nexus chooses not to touch the grey area at all.
There are plenty of public mods here with ported content that you can still download. However to get the better ones you need to work your way up here in the community. The permissions is a hold over from the VGU days where leeching was a big issue. Now it is a demonstration of trust. Many people will download the content and the release it elsewhere against the Mod Creators permissions. Once again, the grey area. By locking the mods away behind a wall it further protects GUN as it is not easily accessed.
If you want access to the better mods, gain our trust. Be active, contribute, post meaningful thought out posts, practice the golden rule, and you'll eventually get promoted. It will take time, be patient. Remember post count, length of time here, and reputation won't necessarily get you promoted. Quality over Quantity. This is a great place to be though and mods aren't the only thing offered here. Some great people inhabit these forums. Get to know them.
As the saying goes here, come for the mods, stay for the community.