Because the tooling is top notch while the language is more flexible/expressive than say C#/Java. Async/await future/observables are built in to std lib, dom A

Because the tooling is top notch while the language is more flexible/expressive than say C#/Java. Async/await future/observables are built in to std lib, dom API is wrapped up in to them and fully typed, etc. It's not an academically revolutionary language - it's just a really solid practical language, with many modern ideas implemented from start, good tools and should be immediately familiar to C#/Java/TS developers. It has tradeoffs but IMO it fixes so many annoyances in TS/JS.