Because NV's game-ready driver yesterday has Black Ops 3 AND Anno.
AMD has just Black Ops 3.
Gamers don't care that Anno or Fallout 4 is NV sponsored, imagine if AMD performs poorly in Fallout 4, who do you think gamers will blame?
Gameready drivers have minimal effect anyways. Most of the times real effect player would really notice is crossfire/sli support (or lack of it) anyways. Ofc, sometimes you might get real performance benefits, if there really is something badly wrong that needs fixing, but most of the time it's just swap of few shaders to get that extra 1 fps.
Seems these new drivers already contain profile for Fallout 4 (hasn't been mentioned in notes). Now if AMD don't release specifix driver update for FO4 when game gets released, they will get shitstorm about it, even they already do have profile for it.