We wanted to stay as close to the standards as possible. Babel was and is still the best tool for that. Switching to TypeScript would take some effort because TypeScript would put additional constraints on our codebase and some of the ecosystem is still not as mature as Babel. I witnessed the Angular 2 team move from Traceur to TypeScript...and the many months that process took them when they thought at first it would only take a few weeks.
We are constantly evaluating this though. What was the best choice when we started may not always be the best choice. We have a good relationship with the TypeScript team and it may be that in the future it makes more business sense for us to move to TS. I don't know just yet, but the arguments for that do get better as time passes.
We do make efforts to have good TS support and I think it's a great choice for building applications.