The difference with Java applets from back then is that wasm was designed by web browsers in order to work well in them. Java applets were plugins, which led t

The difference with Java applets from back then is that wasm was designed by web browsers in order to work well in them. Java applets were plugins, which led to security issues, and worse, security issues not under the control of the browser. Wasm is a proper part of the browser. Java applets had limited interop with JavaScript and HTML. Wasm is also somewhat limited there, but it was designed to at least have fast calls to JavaScript itself. Java also has language-specific issues. Most native code that people want to run on the web is not written in Java or another JVM language. Wasm was designed to support compilation from C++, Rust, etc. (and it has recently added Java support too).