This is tangential and possibly too open ended to be productive but worth a shot anyway. Why is Java so popular? I know of a major Silicon Valley company that's migrating their backend to it. Why Java over other languages? Or maybe there's not really other viable options? I'm speaking as someone who spent the last 6 years focused on frontend web technology.
First comer advantage. Initial Java was a robust, yet simple, sane build tools, relatively safe, typed language when most everything else was c or c++ and not as "safe" (modern c++ is actually very safe language if you stick to its guidelines however). Over time Java has become almost as complicated as Modern C++ but people picked it up over time mostly. It's still used because it's still a robust general purpose language, tons of libraries, huge corps don't like to rewrite things, and there are lots of Java programmers out there. Don't let the webstack blind you, there is a whole other world out there of backend, embedded, finance, and legacy stuff that hackernews audience is just a small part of.