I'm not sure I understand: WebAssembly is the output of a (hopefully) optimizing compiler. LLVM is such a compiler backend. If you use WebAssembly today, you are probably going through LLVM.
Perhaps you meant: why not use LLVM IR instead of WebAssembly? If so, allow me to refer you to this comment[1] (from a bit further down in this thread).
Perhaps you meant: why not use LLVM IR instead of WebAssembly? If so, allow me to refer you to this comment[1] (from a bit further down in this thread).
[1]: https://news.ycombinator.com/item?id=16586239