More seriously, though, I think a lot of the reason for the historical tendency towards server side rendering has been:
* DOM Manipulation is hard and inconsistent between browsers: Yes, JQuery and friends hide the native DOM API. But we have to remember that it wasn't always the case. Rich client-side libraries are all quite young, comparitively.
* Offline capabilites were non-existent: Even today, the HTML5 specification, as implemented, only grants you a fraction of the data storage and retrieval capabilities available to even the most sandboxed application. And previously, these capabilities simply weren't there - if your app needed to store data, it had to store data server side and tag the client with a cookie or some other for identification to keep track of which data belonged to whom.
Yes, there is progress being made on all three fronts, but you can't expect developers to throw away years of best practices in a day, then immediately turn around and write HTML5 webapps. I think the evolution towards a more balanced computing model (where more of the computational load is being handled by the client) is ongoing, and will accelerate as browsers become more capable.
On a related note, the last couple of weeks I've been working on rewriting an old (2000-era) web app, originally written in Visual Interdev. Visual Interdev was widely derided back then, it was something only Visual Basic 'programmers' (the chaff of the programming community) would touch. Turns out that many things it did are a lot more popular nowadays - client side calculations, validation, dynamically updating the UI, etc. Of course there was no XMLHttpRequest, so by modern standards it was quite limited; still, it's funny how something so derided back then just turned out to be ahead of its time.
These were the days. Funny how some things change and "silly ideas" are all the rage.
This is because of the adoption of the technology and its evolution. It is not just the technology per se.
If a tree falls in the forest and no one is around to hear it does it make a sound?
What we are really discovering here is that we can do software that uses the (big) network.
The opportunity is in that, differently from the past, this time we suck less at the UX design.
Actually, it was Apple, not Google, that kicked off the JS performance wars. Apple and Mozilla were deep into JIT tech before anyone knew about the existence of Chrome.