That's a completely fair question. I believe it's mainly just that Java bytecode is optimized to run on the JVM whereas WASM is optimized to run on CPUs. https://github.com/WebAssembly/design/issues/960
Java bytecode is tightly coupled with the JVM.. it doesn't make sense to consider one without the other. There's bytecodes such as invokevirtual or invokeinterface which don't make sense unless you have Java objects and classes.
The benefit of WASM is that it's more general purpose because it's lower level. It's agnostic to your memory model, so you can have a not-Java object model if you want.
