Because using javascript is convenient and fits well in an environment of "just type in a URL and you're good to go"; your users wont have to install anything,

Because using javascript is convenient and fits well in an environment of "just type in a URL and you're good to go"; your users wont have to install anything, you'll be able to make updates EVERYONE will get the next type they visit, and you're not limiting yourself to a fraction of the people who have an up to date version of some plugin. To me that's seriously about the most perfect app deployment mechanism I could want. At least on the web it is. I personally still wouldn't use node.js ever, but I see how it might be used by someone who already knows javascript and doesn't have the time to pick up Haskell or Erlang for some idea they want to execute. Also, it used to be that without the HTML5 tech a lot of what you listed wasn't even possible and that was a forcing factor to use something else. So it's not just improvements in speed here, that RAGE demo wouldn't exist without WebGL.