If I had to guess, it's probably because most of them know he won the award because of technological reasons rather than the story elements.
I'm not posting that to start an argument over the story and plot. But, no one can deny that the movie is as big as it is because of its technology and special effects. That's the forefront of this film and I'd bet that most writers and/or directors feel that their superior stories were unfairly dominated by the big budget monster.
Right or wrong (I don't care), it's an understandable feeling for those other directors at the award show.
edit: I think some of them might have felt the same way when Return of the King won the Oscar for Best Film Of The Year. It was the big budget monster that told an old classic story that people love. The Academy basically made it obvious that Return of the King was going to win because they purposely avoided giving the award to what most feel were the better first two parts of the movie. But the Academy wasn't going to let Lord of the Rings sweep the award three years in a row. Return of the King was basically an honor for the entire trilogy.

Comment