I am in the same boat as the poster. The main reasons I use typescript instead of scala.js are the vast amount of definition files already made (the scala.js translator of these leaves a lot to be desired), and the fact that it's closer to the es6 community which makes maintenance, contributions, and adoption much more likely. So basically because you stay closer to the JS world.

The only downside is lack of "isomorphic" code reuse which I gladly accept anyways because I rarely see server and client side code reuse as beneficial beyond simple models and validation.