It's a reasonable language choice, and the OCaml's type safety was obviously an important factor. However, the key reason why it was such a brilliant choice wa

It's a reasonable language choice, and the OCaml's type safety was obviously an important factor. However, the key reason why it was such a brilliant choice was for attracting top talent. People who show the initiative to learn a functional language tends be very self-motivated, intelligent, and passionate about their craft. As a result, Jane Street has long been known as one of the (if not the) top places to go if you want to do functional programming full-time.