The author, Paul Biggar, was the founder of CircleCI which does use Clojure(Script). I don't know if he has written on why he chose not to use the same stack for Darklang.
Can read about how CircleCI's continued use of Clojure in 2017: https://circleci.com/blog/tips-for-optimizing-docker-builds/ And again in 2019: https://circleci.com/blog/update-how-circleci-processes-over...