Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
Perhaps it's because a talking tree named Old Man Willow just wouldn't fit in with the darker turn the trilogy took in comparison to the novel.
Source: https://collider.com/lord-of-the-rings-characters-not-in-movies/