Not to short-circuit the joke, but in this case, it’s because the valid JavaScript version is…

let a

…and one of TypeScript’s main design goals is to be a superset of JavaScript, that only adds syntax, and doesn’t re-write it.

Beyond that, it’s probably a case of some new language just using what the designer is familiar with.

First time i used let it was to inline variable declaration with assignment . Can’t remember the language.