However, to answer your “why” question: mostly just because I write Haskell for a living, and my primary personal motivation for writing this blog post was to share with my coworkers and to point to during code reviews. Therefore, it makes the most sense for the examples to be in Haskell! Haskell is also especially well-suited for communicating this sort of thing if you already understand the notation, as you can sort of see from the Java translation: it’s significantly more verbose just to get the same idea across. Still, it’d be great if someone were to provide a “translation” of the same ideas to other languages, no doubt!
Haskell is in fact a programming language, despite what the phrasing of your comment might imply. Translation from language to language is often difficult, we don't do typically do it from English to say Spanish not because it wouldn't be valued but because we may not have the same expertise in Spanish and it would be expensive to hire a person to do it. Expertise is important, because key ideas may get lost in translation.
Can't recommend using this library enough; being able to essentially reject any input (e.g. json over http) at runtime because it does not conform to the type definition is such an amazing thing!
Come learn Haskell! It's fun and edificational.