I am unsure, but I can make a guess. The first release of Wasm has been always considered a Minimum Viable Product, which a strong emphasis on Minimum. Pretty m

I am unsure, but I can make a guess. The first release of Wasm has been always considered a Minimum Viable Product, which a strong emphasis on Minimum. Pretty much only features already part of the legacy asm.js "standard" made the cut. The fact that WebKit had some level of support for tail calls on the JS side is exactly the reason we choose it as the right platform to invest in.