Rush to client-side in web development

Question

In the past few months, I recognized a big excitement about client-side scripting in web development. But while server-side technologies are mature, stable and well accepted by backend-developers, client-side technologies are immature (i.e. compared to big server-side framework) and disliked by many long-established developers. Nevertheless everyone is doing client-side development these days. I personally expect those big server-side frameworks to disappear in like 2-5 years, watching the current trend.

Why is that so? How could the new and "diffuse" client-side developing in HTML5/JS possibly be superior to big and well thought server-side solutions?