To be fair, neither Critias nor Timaeus (the two books in which the myth of Atlantis appears) is a Socratic dialogue, although they are both (technically) dialogues in which Socrates appears. In each case the dialogue form is not really used, and the bulk of the text is a single speech by the main character. These books aren't intended to portray philosophical discussion, as in the more familiar Platonic dialogues, but to express myths, something that Plato does frequently but normally by embedding the myths within discussions rather than making them the focus of the entire book, as he does in these two cases.