Because each Bethesda game needs an specific "Script Extender" made by the community. Skyrim have the "Skyrim Script Extender" and FO4 have the "Fallout4 Script Extender". When a game gets a big graphical update, the script is heard to be very difficult to update, cuz is 32bits, thus leaving the mods not working at all.
The Skyrim community almost got destroyed after the release of the Special Edition, cuz the graphics got uodated at high quality, with 64bits, much like what FO4 is planning to do. Fortunately, Special Edition was released as a separate game from the original, and nothing bad happened since the community could enjoy the "old" mods in the original game.
Now, the recent news dont mention if this new Fallout 4 patch will be another full re-release, so FO4 mod fans fear they will suffer the same fate that the Skyrim modding community almost got.