> I can't imagine there being a huge difference between Rust and Go both being quite new.
This is a failure of your imagination. The linked article points out some of the differences, but there are many others. This topic has been endlessly discussed, and I'm kind of sick of repeating it, but I have written several posts on this matter and you can hunt them down if you wish.
Java and in particular the JVM has a massive ecosystem which ranges from small libraries through to major infrastructure components e.g. databases, big data, enterprise, middleware, integration etc. And the fact that the biggest and best companies are using it means you can leverage some high quality components e.g. Twitter, Netflix, IBM. That said all of the above isn't mandatory for 90% of websites most people build.
As for why Java is popular for web server deployment. Not having to worry about memory management, having a less complex feature set and excellent performance are the key features I can think of.