I suppose it's because ROTK is the book that know the best. Well, not really know, but visualise the best. I know what Shelob's lair looks like, what the tower on the other side's like, what the landscape in Mordor's like, and how it's full of orcs and scary even when the orcs are drawn out. I know that Gondor is surrounded by farms and things, rather than being in the middle of a barren plain where it's really difficult to get any food to feed the town from. And I know that the grey havens are not full of buildings that people have built and let fall into ruin.
(What's with the building buildings and letting them fall into ruin in any case? Everywhere except for the shire seems to have more building than people to live in them. Or maybe my people to space ration is just warped by living in London ;)
And it isn't just that - particular moments have been left out. In the book, Sam makes the choice to leave Frodo. In the film, Sam's chased off by the orcs. In the book, Dem ... what his name .. the steward. Anyway, he chooses to die in the fire. In the film, he decideds to set himself on fire, then run off the edge of a cliff as a human fireball (WTF?, incidentally). In the book, Gondor is saved after a long, hard fought battle. In the film there's a tough fight, then along comes ...
Miracle-dead. Cleans up even the toughest armies. You having difficulties with an army you just can't get rid of? New Miracle-dead wipes them off in an instants. Not only that, New Miracle-dead's formula leaves even less residue. Just tell your undead army that they're free of their oath, and they vanish away. New Miracle-dead. Availiable in paths of the dead near you.
Ah, well. On the other hand, at least it matched the first two films. It is a fitting end to the trilogy.