From Lucy: The Beginnings of Humankind by Donald Johanson and Maitland Edey, p. 288, a very good concise description of why Johanson and White did not choose the most complete of the Hadar skeletal remains as the type specimen of Au. afarensis:
Australopithecus afarensis was agreed on.
Next there had to be agreement on the type specimen—the specific fossil that would be used to describe the species. Several of the people in my lab urged me to use Lucy, on the ground that her magnificent condition would make possible an exceptionally detailed picture of the species. I refused. I pointed out that Lucy was not typical of the collection, being extremely small. Furthermore, a complete scientific description of her—a necessary prelude to naming—had not yet been published. I recommended LH-4, the best of the Laetoli specimens. It had been fully described by Tim, illustrated and published.
That would draw Laetoli in, and that was important. Having convinced ourselves that the two collections represented the same type, we felt we should formalize that. The best way to tie them together was to award the species name to one place and the type specimen to the other.
An interestingly significant page number, that…