I think one problem is that it's actually a lot more expensive to build an interactive application-style site that works with well with javascript disabled. So sure, rich companies like Github, Twitter, and Facebook can (and I think should) invest in making things work relatively well. But a less rich company is faced with a choice between giving up that interactivity or committing to it. Most people have gotten used to the interaction patterns of javascript-heavy sites, so giving it up can put a company at a UX disadvantage, with no real pay-off, because it is very uncommon for people to turn off javascript. On the further end of the scale are applications like Google's inbox, which are largely designed around interactivity of the sort you need javascript to achieve, and are never going to work any other way (though it's very silly that it requires Chrome).