Because it has mechanisms in place to make server side driven templates feel like they are client side for most use cases. I carefully included "such as rails"

Because it has mechanisms in place to make server side driven templates feel like they are client side for most use cases. I carefully included "such as rails" because I'm not saying only rails can do it. I can't speak for other frameworks but rails has turbolinks and also seamlessly handles the ability to send back pre-compiled html results to the client as a response to an xhr. Basically you get the best of both worlds and it's all unobtrusive so all of your content is crawlable and you get a great user experience without having to do hacks or extra work.