Why is the quantifier of poetry the first?

Poetry, like articles, can also be called "articles" and "chapters"; For example, The Book of Songs is also called 300 Poems, and Qu Yuan's group poem Nine Chapters and so on. In the Eastern Jin Dynasty, there was a poet named Sun Chuo, who wrote in the Preface to Poems of Sorrow: "Poems of Sorrow are hard to write". Since then, "shou" has been widely used as a quantifier in poetry.