Symbolic reasoning involves a lot of term-matching and term-rewriting. These tasks are best accomplished in programming languages that have first-class support for recursive terms (aka. algebraic data types). Popular candidates these days are OCaml, Haskell, Scala.

While I am not certain the premise holds (OCaml dominance), one possible explanation is that many programmers were introduced to ML/OCaml through the compiler construction course and Andrew Appel's book.