Fallout 76 hasn't got official mod support yet, in case you didn't know. While a number of mods may have been released when the game came out, most of them were simple reskins, some of which even got people banned!
With that in mind, why is it that Bethesda hasn't yet implemented Fallout 76 mods properly? The answer is actually precisely what you'd expect it to be, as it relates to the game's multiplayer nature.
Even though Bethesda themselves announced Fallout 76 mod support to be out before the end of 2019 - even before the game was actually released - it seems unlikely that this will be the case.
According to Chris Mayer of Bethesda, the fact that Fallout 76 is a multiplayer game makes mod implementation way more difficult: "(Fallout 76 multiplayer) mandated that we changed everything from the scripting engine to how the systems work, because now you have round trip from the client to the server to get it validated by the server and back to the client for most of our systems."
Vaguely enough, Mayer was optimistic: "So it’s on our roadmap," he said. "It’ll be a step-by-step process as we lay it out. It’s something we definitely want to do."
Do keep in mind that we're first going to have to gain access to private servers and whatnot, which is a long way off as well. In other words - don't hold your breath for Fallout 76 mods just yet.