According to Edgar Allan Poe in the philosophy of composition why are short poems preferably when it comes to unity of impression

Because they can be read in one sitting and the unity stays intact

Explanation:

Edgar Allan Poe once mentioned that short poems are better for being not as complicated when compared to a long one. Since it can be read at once with much less effort, it is also, usually, more memorable, causing it's impression to stick and being able to keep a single idea from start to finish without being repetitive, making it much easier than a long one.

Because they can be read in one sitting and the unity stays intact