JavaScript is a valuable target for transpilation because it runs in all web browsers. Some benefits of using other languages:
Supporting ES6 (and beyond) in a browsers that only support ES5. See: BabelJS
Using languages for web development that are easier to use than JavaScript or otherwise desirable. For example: CoffeeScript, TypeScript, Dart.
Re-using large amounts of legacy code with minimal effort. Analogy: This is like building an automated translator from chinese to english as opposed manually translating every chinese text to english. Example: Unreal Engine
JavaScript is a valuable target for transpilation because it runs in all web browsers. Some benefits of using other languages: