The first reason is due to Edgar A. Poe's writing style, which was formal in nature. Poe was a master of words and had an extensive vocabulary, and he made use of it in his writing. His style is instantly recognizable, just as Hemingway's or Faulkner's would be to most people who are well read. The other reason is because this diction Poe uses would be indicative of the character of the narrator. The narrator was a well-to-do man who was upper-class, so his diction would be formal, unlike someone who was from the lower classes. The narrator is also very calculating and careful, so one would suspect his diction would also be this way.