On the browser side: What killed Java was to a large part loading times. First the Plugin had to load, then the bytecode had to be run. On many systems one imm

On the browser side: What killed Java was to a large part loading times. First the Plugin had to load, then the bytecode had to be run. On many systems one immediately knew when Java was used by the browser getting slow and sleeping for a while. Flash loaded a lot faster (also systems were better, generally) however Flash apps completely messed with user experience. Nowadays JavaScript can do a lot of things better (say changing URL, history support, back button) which can be integrated with a wasm tool for having a way more seamless integration. As a user you simply don't notice if something is using JS or wasm. From there it imo carries over to the server side and other places. Java simply got a bad reputation as resource hog used for bad UI in Applets and many people looked elsewhere. And then wasm supports C and C++ (and more) with huge eco system of libraries, applications, ... (While of course these days a Java VM (incl. Android) is often used with non-Java languages as well)