This is just my personal viewpoint with nothing to back it up, but I doubt Google ever seriously wanted other browsers to implement PNaCl. As this post says, it was never specced and Google's big push for it was in Chrome apps on the Chrome Web Store. Also, wasn't asm.js just a subset of JS? This line confuses me: > asm.js and PNaCl represented quite different visions for how C/C++ code should be supported on the Web…
WebAssembly (at least to start with) is mostly intended as a more compact binary encoding of asm.js. It still mostly executes the same way and offers the same APIs as asm.js.