There may be several reasons why Wordsworth chose the word daffodils, which first appears in line four. The final syllable rhymes neatly with "hills" in line two. The rhyme pattern is repeated as end rhymes in lines twenty-three and twenty-four with "fills" and "daffodils." It works well metrically, too, with its three syllables that contribute to the poem's consistent iambic tetrameter.
Beyond the mechanics of the meter and rhyme scheme, the choice of daffodils works well with Wordsworth's subject matter. Daffodils bloom in the spring and are often colored bright yellow, a color associated with the happiness that the poem's speaker feels when he remembers how they looked. Daffodils naturalize into a landscape and spread, which makes the idea and image of them growing in a vast field completely plausible.