Because the historical position of the Sui Dynasty is very prominent.
Sui dynasty was very short, and its achievements in literature were not high. Poems before the Song/Qi/Liang/Chen Sui Dynasties mainly described court life and were called palace poems. These poems describe the charm/posture of women, and their aesthetic taste is not high. China's poems can best reflect the life of the public and the sufferings and joys of the working people. The Sui Dynasty and even the early Tang Dynasty were influenced by palace poems.
However, there is a farewell poem by anonymous in this issue, which is quite fresh.
Willow green hangs down to the ground, and flowers are long and sultry. Willow branches are broken and flowers are flying. Will pedestrians come back?