For the same reason people don't write every program in binary using a magnetized needle straight to their hard disk. People like abstractions, and this guy happened to like CoffeeScript as his abstraction du jour.
The fact that it uses whitespace instead of braces is a big plus for people used to that, and generally you write less coffeescript compared to TS or ES. Plus it has things like switch expressions, list comprehensions, chained comparisons, the safe nav operator, and a ton of other little features that either don't show up in TypeScript or ES.
And it's very simple (no configuration needed), and compiles to very readable javascript which means converting to plain JS is very easy (hell, there's even a purpose built tool called decaffeinate which will translate it directly to ES2015)
You might not like the language, I don't really like it all that much anymore either, but it's not irrelevant, and for a small personal project like this it seems like a great choice which is easy to switch away from if needed.
