DA, the reason ROTK was omitted is simple:
The film was a technical and historical achievement (representing the trilogy as a whole) and is one of few films to match tremendous critical success with even greater commercial success. And while a few critics may have wanted to see Coppolla's Lost in Translation take the crown, FEW (and I mean FEW) believed Mystic River or Master and Commander deserved Best Picture over King.
The other five winners, however, were all hotly debated after the awards. Did Gladiator really deserve it over Traffic? What about Chicago over Two Towers or The Pianist? Crash over Brokeback? Baby over Aviator? And Mind, which wasn't even the third best reviewed film of its year?