Tree processing is best done in a language with decent algebraic datatypes and pattern matching. I would’ve preferred Standard ML, but, well, pot-ay-to, pot-ah-to. Haskell is another choice but the techniques you need to use there (while undeniably gaining you some possibilities) don’t really generalize to other languages, so you’re now writing a book about compiler construction in Haskell rather than just compiler construction. Ditto for Rust. Kotlin has deliberately anemic pattern matching. C# or F# leave you depending on Microsoft’s benevolence (sic). Metalua and Sweet.js both have decent ADT support but both are pretty much dead. Racket exists, I guess, and there are some pattern-matching libraries for normal Scheme as well, but the charisma malus of the parenthesis is real even if I don’t understand what causes it.

So OCaml was probably the most mainstream choice among the languages with appropriate tools, as funny as that sounds. And honestly, once you get over the syntax, it doesn’t actually have anything outrageous.