The author explains why Jepsen, a tool for testing concurrent systems, is written in Clojure. Clojure's immutable data structures, concurrency support, and Java interop make it a suitable choice for the project. The author also mentions the language's conciseness, adaptability, and prototyping capabilities as advantages. Despite some drawbacks, such as the lack of a static type system, the author believes Clojure was a good choice for Jepsen.