It seems pretty clear that it's for practical reasons: you want that sort of rich in-browser experience, it's going to be easiest to provide using JS.
What I think is a more interesting discussion is why they're not using CoffeeScript or some other flavor of JS that glosses over a lot of the idiosyncrasies of the language and arguably offers a more beginner-friendly syntax.
What I think is a more interesting discussion is why they're not using CoffeeScript or some other flavor of JS that glosses over a lot of the idiosyncrasies of the language and arguably offers a more beginner-friendly syntax.