Because TypeScript is the Default language of Angular2.

To go along with @ithinco 's answer Angular is more of a framework and therefore a complete solution whereas React is not. It's simply the view layer that can be used inside just about any UI stack. So React has been kept more agnostic from such bindings.

Angular was developed for designers at google who needed to work with persistent forms. And the development was performed in AtScript which is a super set of ES6. Typescript developed from there and some of the developers actually refer to it as annotations and or 'types' (type descriptions) for JavaScript.