* it's standardized
* it's easy (yes, it is, period)
* it's f*ing included in each and every browser, and its rise followed closely that of the web.
Moreover, the fact that Adobe came with a more powerful yet proprietary VM (namely, Flash) forced evolution of Web standards (JS included) so they could live on. That's Darwin 101.
Every use of JS outside the browser comes from developers willing to have the same ease of use out-browser than they had in-browser.
But JS, as popular as it is, is not yet as popular outside of the browser than it is inside of it.
Stop looking at the Web ecosystem and dive into the industry, you'll see.
What will happen to JS if we kill the browser?
Or what will come out that will kill the browser? And will it have an impact on JS?
The bond is yet very tight between those two pals, don't you think?