Did a bit of digging and found that there used to be a comment for why it was done, but it got removed [0] when they switched to the implementation from Bentley & McIlroy's "engineering a sort function" [1] around 1992.

[0]: https://github.com/weiss/original-bsd/commit/d3fcf71e0db57cb...

[1]: https://cs.fit.edu/~pkc/classes/writing/papers/bentley93engi...