The reasons turn out to be decently boring here. Zoncolan [0] and Pyre [1], which Pysa shares core libraries with, are also written in OCaml, and the language made sense to use from the perspective of both sharing code and having people who are proficient and comfortable writing in OCaml working on the project.
[0]: https://engineering.fb.com/security/zoncolan/
[1]: https://github.com/facebook/pyre-check