Yeah, WASM was the result of taking a look at NaCl, realizing it could never be specified and independently implemented, and writing a spec for something better that could.
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.
(P)NaCl was just too close to the past, roo many memories of activeX, flash and other crap still loomed heavy in people's hearts and they wanted a clean cut with the past, an open web, free from the dictat of corporations etc.
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
I used to work in the Native Client team at Google, both on regular NaCl and pNaCl. It failed for technical reasons too. And maybe, mostly for technical reasons.
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.
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.