At the time, asm.js was the standard for high-performance web applications, and Google decided to go with a format only they used, with no backwards compatibility to any other browser, ignoring the back then WIP WebAssembly.
And despite using a simple compiler backend to compile their native code to NaCl that also allows support for asm.js and WASM in a matter of a few days (entire game engines have been ported that way), Google has been going for months and still kept it Chrome-only.
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.
And despite using a simple compiler backend to compile their native code to NaCl that also allows support for asm.js and WASM in a matter of a few days (entire game engines have been ported that way), Google has been going for months and still kept it Chrome-only.