The writer, Edgar Allen Poe, chose to purposely not give the narrator a name. This gives the narrator credibility in his meeting with Roderick and the events leading to the downfall of the family name. He can account for how he knew Roderick as a boy and how he has changed so many years later. He can attest to his gradual downfall just through the short amount of time he has spent visiting. This story is written from first person point of view and all of the characters and occurrences are described by him.
hmm...that's a good question. I've always thought that Poe didn't give the narrator a name because he wanted him to represent rationality. Without a name, and with very little biogrphical detail given (all we really know about the narrator is that he was a childhood friend of Roderick) we can assume that the narrator is objective. However, you could just as equally argue that since we don't know anything about the narrator, that he in fact isn't rational, and that what he sees/tells the reader isn't reliable. Some have suggested in fact, that the ending is just a figment of the imagination of an unreliable narrator. I believe the former, however, because there is nothing in the writing to lead one to believe that the narrator is anything but objective and reliable.
The narrator is not given a name purposefully in this story. Poe does not want the reader to focus on the narrator. He is not a main character but only one who tells the story of the main characters. Roderick and Madeline are the central characters and Poe wants them to be solely focused upon. If the narrator was named the reader could then focus upon him as a person and the focus would not be on Madeline, Roderick and the house.