I think the real reason react is taking off is the ease of integration with es6. It's weakness is the lack of support for web components. The lack of support may cause react to lag behind dart and angular 2 in the future but right now it's a nice way to work with es6 and get some pretty good code organization. Angular 2 is going to be easier to test but people also use react with typescript for the same level of testability. But ultimately the popularity has to do with the way react integrated with babel and es6.