Answer:
Because they can be read in one sitting and the unity stays intact
Step-by-step 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.