I would say its just a matter of enough developer mindshare, combined with node.js invigorating the JavaScript community, combined with html5 hitting its stride. The fact is that these libraries are certainly not the first, even if they do have a sort of new breed feel to them.
Not that it matters because it's still closed source, but my company created a rich client side JavaScript framework with many of the same ideas as the ones you mention - client side templating, data binding, MVC, REST based data services, automatic lazy loading, build tools, etc. And we've been doing it since 2008. Other frameworks have certainly cropped up in that time period as well before backbone, too. JavaScriptMVC has been around forever, and sproutcore too.