[curveship]

The TypeScript team's philosophy* is that they want to incorporate features that have either ~0% or ~100% chance of making it into ES. Types they put in the 0% bucket. Once a feature is near 100%, they'll include it. But if it's only at, say, 60%, it doesn't make the cut, for fear that it will change before standardization, leaving TypeScript in the "annoyingly different" situation you describe.

* - [edit: found the source] "We'd really like to hit features that have either a ~0% chance of making it into ES7+ (e.g. type annotations), or have a ~100% chance of making it in with easily-defined semantics (e.g. where fat arrow was two years ago). New operators are likely to fall in the awkward middle." [https://github.com/Microsoft/TypeScript/issues/16]