SPA applications frameworks and architecture are so young compared to server side rendering and still evolving. Why do they exist? User expectations of the web

SPA applications frameworks and architecture are so young compared to server side rendering and still evolving. Why do they exist? User expectations of the web are increasing and waiting for server responses from any action isn't really cutting it anymore. Of course it's harder to maintain state and develop - the patterns haven't been clearly defined, and SPAs introduce two sources of state (client and server) which is a hard problem. It's the cost of the demanded user experience. Angular ui router, ember data, react flux architecture are all examples of things that have just started evolving to address these kinds of issues. All of these are very new and being constantly iterated on and updated. Angular ui router does a good job of handling things like the back button and managing the state of your page. Ember data has really powerful client side data state management, and react flux architecture presents a much needed practice of how to control the flow of data between server calls and views. HyperMedia APIs are another evolving technology for maintaining more control between the client and the server.