One reason (as I see it) is that TypeScript is implementing ES6 features along with the type-specifc stuff, so it's going to end up in a place where it's very compatible with the 'future' of JavaScript.
CoffeeScript will always be something quite different. I love it, but that difference makes me nervous. ES6 ought to bring a lot of the features I like about CoffeeScript into JavaScript.
It is notable that TypeScript chose not to track the ES6 modules standard for the last half a year: https://typescript.codeplex.com/discussions/446695 The modules you are writing in TypeScript are not compatible with ES6.
The TypeScript language team has yet to prove that they will track the specs and implement features as spec'd. I think claiming that they will "implement ES6 features" is a bit pre-mature. So far, they have implemented a few drafts and ignored a few others. A mixed bag.
I don't want to troll here but "very compatible" sounds like MS best days of EEE - Typescript still IS a different lang. Question is what if future Javascript will go in a different direction than todays Typescript ?
Typescript main feature is tooling but I would rather go with lang where I'm not second class citizen on non-windows (eg. Dart's editor works on Windows/OSX/Linux)
Do you really feel JavaScript has that luxury? Seeing as how widely deployed and used it is I feel that it can't go in a completely different direction anytime soon.
CoffeeScript will always be something quite different. I love it, but that difference makes me nervous. ES6 ought to bring a lot of the features I like about CoffeeScript into JavaScript.