The poem "Lilacs" is written in free verse. The poem flows almost like a conversation. The poem doesn’t follow any sort of traditional pattern or style. For example, the lines of the poem vary in length and do not follow a rhyme scheme. The free verse form allows the poet to create vivid and descriptive images.
If the poem had been written as a sonnet, the poem would lose much of its description. The 14-line structure of the sonnet would restrict the poet's words and ideas. As a result, the images would have to be much less detailed and developed. For instance, consider the following lines from the poem:
Lilacs,
False blue,
White,
Purple,
Color of lilac,
You have forgotten your Eastern origin,
The veiled women with eyes like panthers,
The swollen, aggressive turbans of jeweled Pashas.
These lines give the impression that the speaker is looking at and addressing each lilac separately—each line represents each lilac. The sonnet form would not have allowed for such varied line length or involved descriptions.