Did you read the article I linked? It explains why they went with NaCL over ASM, and why, despite them having a WASM prototype for months now, it's not ready for production yet.
This isn’t the first time Google has released a product exclusively for Chrome, trying to pull more users to their own platform. Even if this is not directly intended, the result is a massive anticompetitive effect.
Basically: native threading. NaCL supports it, ASM and WASM don't (yet).