Along the same lines as the award by the Writer's Guild of America, this seems less like the result of a truly thorough analysis, and more like the National Board of Review just really wanted an excuse to award this movie.
This is not said to imply that Cate Blanchett's performance as Galadriel was bad in any way, shape or form. On the contrary, she actually did quite well given the limited amount of material she was given.
The problem is exactly that: The limited material she was given. Throughout the entirety of the film, Blanchett is on camera for somewhere around ten minutes.
Be honest: Unless you're a die-hard Cate Blanchett fan or a proud nerd who could even name the actor who played Isildur, what are the chances that you even remembered beforehand that Blanchett was even in the first of the Lord of the Rings movies?
2001 was by no means a slow year for acting. This was the year that gave us Halle Berry in Monster's Ball, Judi Dench in Iris, and Renée Zellweger in Bridget Jones's Diary.
That's not to imply that their performances, or the movies in which they appeared, were undeniably better than Cate Blanchett or The Lord of the Rings.
But they were full performances, which really should have been the NBR's first criteria when deciding whom they would give top honors to.