(this won't be quite true after TS 2.0, but..) TypeScript doesn't support everything ES6 or even everything Babel does, and some of the stuff it does support are not matching the standard, in subtle ways. Then you're missing out on the Babel plugin ecosystem. You can use TS and Babel together for SOME of that (but not all of it, syntax parser and all).
It may still be a slam dunk to you, but it's not as "obvious" an answer as you make it sound.
It may still be a slam dunk to you, but it's not as "obvious" an answer as you make it sound.