Rails was designed at a time before angular brought the SPA into fashion. SPAs manage state and template rendering on the front-end, so much of the Rails machinery becomes unnecessary. You can still use Rails to implement your JSON API backend, but the simplicity of something like Sinatra is often preferable since you're increasing the complexity on the front.

The node+react+webpack combo gave web devs the power to process application states and render the UI on the front-end and the back-end using the same code; a feat that is not possible at all with Rails pushing it a little further out of fashion.

Of course, the SPA suffers plenty of criticism, and Rails is still a popular choice with and without an SPA on the front-end, but I think it's fair to say that Rails is no longer the "fun" way to do things (though the most fun doesn't mean the best business or engineering solution).