You have to remember that WASM comes from web browsers. It is meant to be transferred across the web and run in a the browser on the target computer. It isn't very practical to compile everything to every architecture/os combination and serve the correct implementation.
Yeah but that's not the context of this use. Why are they taking something made for browsers (where architecture neutrality is important) and using it for sandboxing, instead of faster sandboxed (not VMed) native?
They are doing it for the same reason people keep ripping JS engines out of browsers: there has been so much work done to make them fast, harden the sandbox, and port them to different platforms. It's free real estate.
