If the why was ever recorded anywhere, it would have been in the comments of the code when it was committed, because it wasn't explained in the documentation or anywhere else other than: "it does this". Either that or a email/USENET discussion from that time period; but those aren't so easily retrieved unless you were originally involved in that ancient discussion.

It's worth the effort to document going to the source to see if any insights code be unearthed, even if it doesn't reveal much, just so that others know what has already been explored and to look/ask elsewhere if this is not a sufficient background.

I think the logic to the why has been lost to time.

We might talk about why it would or wouldn't be a good idea for a modern version of ls; it's something to give a few moments' thought, but no more.

It does do the 'why', in a way. Why does the OSX ls do -foo-? Well, because the source OS did it that way. It wasn't a decision by the OSX people, but inherited.