Perhaps it was the unchanging ambient pad that played in the opening scenes. Or that the most memorable piece of music was an arrangement of In the Hall of the Mountain King. Or that art composers don’t care about the the sound design of the snare drum on a particular beat. Or that there wasn’t an orchestra.
I managed to predict this Oscar (although my Oscar pool predictions were generally horrible). My thinking was: Alexandre Desplat (The King’s Speech) A. R. Rahman (127 Hours) and Hans Zimmer (Inception) had all won before. John Powell (How To Train Your Dragon) wrote a very traditional score, and would be at a disadvantage. Nothing electronic-sounding had won an Oscar since Chariots of Fire in 1981 (Slumdog Millionaire is the closest ).
This, by the way, was all without every having seen the movie, and without hearing the actual score.
Then I saw the movie. I have updated my theory. Now I think it won because of the first six piano notes in this clip:
Those six notes summed up the whole film: Isolation. Loneliness.
Just goes to show, you don’t have to write a fugue to express powerful emotions.