React is an important project of Facebook, so FB engineers don't want to be depended on a Microsoft's project (TypeScript). Furthermore, they are smart folks that have a lot of funding to spare, so they want to invent better tools for their development purposes.
Unlike FB engineers, we don't have that luxury. We could rely on whatever open source projects if we would like, Google/FB/Microsoft is ok for us. In my last year's research for choosing FlowType or TypeScript, I tried to favor FlowType, but in the end... TypeScript is the right tool for my job.
Reason is the future of Facebook's projects. Again, that's why FB engineers don't want to fully "support" TypeScript for their top project.