Q. Why is ibid. preferable to id.? The meaning is essentially the same and id. is more succinct, and it is used extensively in legal citation without any apparent confusion or misunderstanding.
A. What you say is true. But because id. (idem, the same) is used mainly in legal citations whereas ibid. (ibidem, in the same place) has long been preferred in history and most other academic disciplines, ibid. is much more well known than id. Whether ibid. also benefits from not looking like id (the complement to the ego and superego) is anyone’s guess.
For more on ibid. and id. (including Chicago’s preference for shortened citations over either of those abbreviations), start with CMOS 13.38.