Mainly due to metrical requirements.
Poetry meter is a high degree of the beauty of form and content of Chinese classical poetry. In form, it pays more attention to the beauty of rhyme and the beauty of antithesis, which leads to the normative requirements of poetry meter.
Modern poetry, also known as vernacular poetry, is a type of poetry. Modern poetry can be traced back to the late Qing Dynasty. Compared with classical poetry, modern poetry has free form, rich meaning, and the management of imagery is more important than rhetoric. The use of it completely breaks through the characteristics of ancient poems that are gentle and honest, sad but not resentful.