As someone who has worked with PNaCL before, I think its really important to stress how utterly terrible it is to work with. The intermediary representation for

As someone who has worked with PNaCL before, I think its really important to stress how utterly terrible it is to work with. The intermediary representation for PNaCL was based off of LLVM IR, which is not (contrary to what a lot of people think) platform independent. The nail in the coffin though, was probably sandboxing. PNaCL is basically just running some opaque binary on your computer and crossing your fingers that chrome's sandboxing was up to snuff. Wasm, on the other hand, is built from the ground up as a mathematically verified sandboxed vm, and this works so well that modern wasm compilers don't even need to emit bounds checks on memory lookups in many cases. On a more organizational level, other browsers were not comfortable implementing something with so much control from a single company.