Because:<p>1. JScript added conditional compilation directly in the browser which was IE-only (extend, extinguish). TypeScript compiles to cross-browser JavaScript (which does none of the above)<p>2. TC39 is supposed to be working in a &quot;pave the cowpaths&quot; (1) mode. Before new features get integrated into EcmaScript, TC39 looks into what the community is already doing (existing cowpaths), then integrates that into the language. Not only that but TC39 can learn from the mistakes of TS and Flowtype when they add type system support in EcmaScript. We are in dire need of one - and thanks to Microsoft&#x27;s and Facebook&#x27;s explorations, we now know what kind of type system would work for JS.<p>(1) For example, we got arrow functions in ES2015 thanks to CoffeeScript.