wasm isn't quite ready yet. It requires a javascript layer for interacting with the DOM, which is slower than plain javascript, and it doesn't support features like garbage collection which would make it a good target for some languages.

Also, javascript is plaintext and doesn't require a compile step, even though for some reason (cough Node) javascript development usually does, which makes it much more convenient for its original intended use case (a scripting language for the web) than wasm.