Why are Tang poetry and Song poetry easy to understand, but the classical Chinese of the same dynasty is difficult to understand?

Poetry often describes a thing or a phenomenon. At present, the poems that undergraduates have been exposed to are relatively easy to understand. In fact, there are many poems in Tang and Song Dynasties that are difficult to read, but we haven't been exposed to such works. That's why I gave you this illusion.

But ancient prose is different. Some words in prose were written according to the language habits at that time. There are many modern times that are no longer used. In fact, ancient poetry and ancient prose are easy to understand and difficult to understand. For example, Liu Zongyuan's Eight Chapters of Yongzhou is a short travel story in the form of prose, which is easy to understand and read smoothly. Qu Yuan's Li Sao, as a poem, is not so simple to read.