Maintaining a browser is already hard enough, it's a very tough sell to convince 3+ browser vendors to implement a new language with its own standard library and quirks in parallel without a really convincing argument. As of yet, nobody has come up with a convincing enough argument.

Part of why WebAssembly was successful is that it's a way of generating javascript runtime IR instead of a completely new language + standard library - browsers can swap out their JavaScript frontend for a WASM one and reuse all the work they've done, reusing most of their native code generator, debugger, caches, etc. The primitives WASM's MVP exposes are mostly stuff browsers already knew how to do (though over time, it accumulated new features that don't have a comparison point in JS.)

And then WASM itself has basically no standard library, which means you don't have to implement a bunch of new library code to support it, just a relatively small set of JS APIs used to interact with it.

Webassembly does not generate JavaScript IR. Not sure where you got that idea. Maybe you're thinking of asm.js?