There are several important lessons to learn from the Java bytecode format and members of the WebAssembly (including myself) do have experience here. In particular, JVM class files would be a poor fit for WebAssembly because:

1. They impose Java's class and primitive type model. 2. They allow irreducible control flow. 3. They aren't very compact. Lots of redundancy in constant pools across classes and still a lot of possibility for compression. 4. Verification of JVM class files is an expensive operation requiring control and dataflow analysis (see stackmaps added in the Java 7 class file format for rationale). 5. No notion of low-level memory access. WebAssembly specifically addresses this, exposing the notion of a native heap that can be bit-banged directly by applications.

See https://news.ycombinator.com/item?id=1894374 from @nix.

Back when Java Applets were a thing, Sun wasn't friendly with browser makers. JavaScript was a gimmicky alternative that was created by a browser manufacturer. It had the foothold, and it grew.

Nos Oracle isn't interested in Web.