The unofficial reason is that Lee gave something away in an interview which seriously ****** PJ (pete jackson) off, hence him cutting out Lee and Douroffs death scenes.
The official reason is that time constraints meant the film had to be cut. But it was only an extra 5 minutes and it was very important to the story to be kept in, which makes one think if the unofficial reason was indeed true.
Lee was asked to come back and do the voice commentary for Return of the King special edition dvd, which he did, and said how disappointed he was that the scene had been cut out, but remained professional throughout.
Also a shame that we didn't see Brad, nor Christopher, and that Bernard Hill lost, what he thought of, as his best scene.