I think it's that, and the general accessibility of Javascript. Before this JS craze you saw the same shit with PHP and Visual Basic, though for the most part people weren't trying to build and rebuild the same platforms nearly as much as now
For me, the most compelling example of progress coming from the JS camp was my shallow investigation of the Expo platform for React Native. It has some genuinely sexy developer ergonomics features which I'd love to see make their way back into Rails.
Hartl is great, has his/its place etc. Sometimes you just need a smart person with experience to add personal context. Happy to be of service.