According to edgar allan poe in " the philosophy of composition," why are long poems a problem when it comes to "unity of impression"?

Poe believed that all literary works should be short. "There is", he writes, "a distinct limit... to all works of literary art - the limit of a single sitting." He especially emphasized this "rule" with regards to poetry, but also noted that the short story is superior to the novel for this reason. 

The essay states Poe's conviction that a work of fiction should be written only after the author has decided how it is to end and which emotional response, or "effect," he wishes to create, commonly known as the "unity of effect."

Once this effect has been determined, the writer should decide all other matters pertaining to the composition of the work, including tone, theme, setting, characters, conflict, and plot. In this case, Poe logically decides on "the death... of a beautiful woman" as it "is unquestionably the most poetical topic in the world