This post is actually a joke, but it does bring about an important point: For an interpreter, having more information results in faster execution. WASM is much closer to Java bytecode than you might think, and SpiderMonkey/V8 are basically the JVM. WASM also undergoes multiple different stages and kinds of JIT compilation in most browsers, and detailed type and usage information helps that produce faster execution.

Also, don't forget that WASM is designed to replace JavaScript, thus it must interoperate with it to smooth the transition. Rosetta and Prism also work to smooth the transition from x86 -> ARM, and much of the difficult work that they do actually involves translating between the calling conventions of the different architectures, and making them work across binaries compiled both for and not for ARM, not with the bytecode translation. WebAssembly is designed to not have that limitation: it's much more closely aligned to JS. That's why it wouldn't make sense to use a subset of x86 or similar, as it would simply produce more work trying to get it to interface with JavaScript.