I can start building an actual app within 3 minutes of typing "ember new" – with React, I'd spend the first couple of days just fiddling around with build scripts.
Sure, there are now react boilerplates that do this, but there are too many out there to properly evaluate what to use. Why not just make it a part of the framework?
I think one of the reasons React has been successful is that most developers inherently enjoy the actual act of tinkering as much as they do actually building something. They are ok with spending the first 72 hours of their project picking and setting up their build tools.
On the other hand, I can't stand that stuff.
That's what's so weird to me about all this. Ember is to front end JS exactly what Rails was. Yet rails was successful and Ember wasn't. I think it's the difference between front-end culture and back-end culture.