(i will try to not repeat those from above)
Why Rails?
(i will be boring here)
stable
fast prototyping
docs/community
convention over configuration
language - just write something in ruby and read it - it’s hard to not understand the code, even for noob like me!
Why with React?
Because it’s not angular 
In react more than in other libs community focuses on how to write quality code thats easy to test, easy to understand, and uses guidelines. Many good ideas came from react ecosystem - functional programming in js, components, 1 way flow, flux/redux.
React is small, so it has low learning curve. It’s much easier and faster to learn than other frameworks. But being small is also disadventage - it needs some libs if you want to do some other stuff 
JSX - you keep all things in one place, you dont have to jump from js file to html file etc. Another good thing about it is that if you fucked up something in template you will get error message (damn you, angular! burn the witches, burn the witches!)
Stability - react api changes not so often. Facebook stands on newest react BETA so they cannot screw it.
Its damn simple js. No decorators, no magic methods, no other crap. With basic knowledge about js you can do fine in react.
React on Rails has it all. Sticked together already. There is no reason to not love it. It does all the hard work for you.
Any comparisons to alternative frameworks and languages?
Angular. I use it everyday, i cry everyday because of it. The bigger the app is, the more it hurts.