Is there really any reason to use Javascript as a layout engine for what is really a traditional forum that we've been doing in HTML/CSS since the dawn of the web? Frankly, it feels slow and clunky to load, as each component loads individually and re-positions everything else. Page switching feels horribly slow as well.
This is a case of single-page-appification of something that really isn't an app at heart, but is simply a content-centric website.
The only reason I can think of - and admittedly I can't prove it but it seems intuitive - would be the assumption that pushing rendering to the browser and breaking requests up into fine-grained AJAX requests or doing resorting in the browser, takes some burden off the server. Why should you have to reload an entire page, for instance, when a thread only contains one new comment? Browsers aren't dumb terminals so let them do some of the work.
Of course this is assuming the javascript isn't also piling on a bunch of nifty effects as well. For me, though, it seems to load really quickly.
You can solve that with React.JS rendering on the server, then again on the client once the page has been downloaded. That makes the page render quickly, then it acts as a single-page app once loaded, without appearing to be jerky and presenting either a loading screen or a half-loaded page which takes multiple seconds to load fully. Twitter do this for their web interface (although not via React).
This is a case of single-page-appification of something that really isn't an app at heart, but is simply a content-centric website.