I've got to admit, I'm really clueless about these things

But don't we already have a universal byte code with JVM bytecode?

Why did the wheel need to be reinvented here? Are there fundamental issues with JVM bytecode that make it impossible/impractical to have c++/rust/LLVM compile to it?

This isn't a troll question. I'm curious what are the fundamental underlying technical issues that made WebAssembly necessary

(please be nice... c++ developer here.. so I'm really clueless about web stuff. I don't even use the JVM)