Then there's the Microsoft factor. Its rocky relationship with open Web technologies over the years has left many Web developers with a sour taste that won't easily dissipate.
TypeScript is, objectively, doing everything right. The project is being developed in the open on GitHub, under an Apache 2.0 license, and it accepts community contributions. The vast majority of pull requests are from the GitHub accounts of Microsoft employees on the core team, naturally, but you can also find community-contributed bug fixes. This is not the behavior people have learned to expect from Microsoft, and it will take more than a project like TypeScript to change those expectations.
More accurate, it's an idea whose time has come back around again. TypeScript's type annotation syntax originated in ECMAScript 4 and was implemented in ActionScript 3, but never landed in the ECMAScript 5 that runs in today's browsers. Type annotations aren't part of the forthcoming ECMAScript 6, but are widely expected to land in the next incarnation of the language. Nobody knows what those annotations will be. But AtScript and Flow are betting that they'll look a lot like the TypeScript syntax, so they're aligning with it.
Sign up for Computerworld eNewsletters.