WASM is superior in pretty much all ways to NaCl other than being a bit more time to go through the whole specification and implementation process. NaCl was a great prototype, and WASM is a much more polished result because of it.
What kept NaCl from being independently implemented? And what did WASM learn from NaCl?
Edit: ah, someone already mentioned NaCl being CPU-specific.
I do understand why people wanted something different and rejected (p)NaCl. But the reason is not technical, it's political (in the broad sense). Any technical issues could have been solved by fixing and extending (p)NaCl, but everybody involved understood it was not going to work
pNaCl's biggest mistake was using LLVM bitcode as a wire format (see, for example, [1]). Another problem was trying to use glibc, instead of newlib. That resulted in a lot of low-level incompatibilities, as glibc itself is too target-specific. And implementing shared objects between portable and native world was really messy.
asm.js appeared as a simpler alternative to NaCl, and then it was quickly replaced by wasm, developed mutually by Mozilla, Google and others.
1. https://groups.google.com/g/native-client-discuss/c/h6GJ8nQd...