Repetition rather than use of synonyms goes well with Wordsworth’s description of what kind of character and language he is trying to embody. This lyrical ballad is meant to describe a pastoral scene in the plain language of a common-man; one who has the imagination to attach symbolic meaning to the physical world, perhaps, without the education or opportunity to do so in an elevated manner. Thus the speaker of the poem, who Wordsworth describes as a common notion with whom the reader would be able to immediately identify, would not feel the need to use synonyms when it is perfectly effective to simply repeat language. Simplicity is the key to the diction and imagery of this poem.
In “The Thorn” Wordsworth acknowledges that the poet can never put into words the depth of human feelings due to the “inadequateness of our own powers, or the deficiencies of language” (Wordsworth, Note to The Thorn). Thus to write poetry in a plain yet expressive form which focuses on simple images to convey meaning is more humble and more pure, because it accepts that even the most intricate language cannot express the complexities and profundity of human emotions. However, Wordsworth is not, perhaps, as humble as he seems. By creating the speaker and his surroundings in simple terms, he is suggesting that the poet himself is the master of the pastoral world he has created, not a member of it.