all engines except Safari's JavaScriptCore seem to have decided that implicit tailcalls optimization wasn't a good idea after all and as far I can see are not planning to enable it.
So yeah, I'm sure I'd be pretty pissed if I were the JSC devs, and not so happy about the community turning around and saying now we want explicit tailcalls. And by the way, they're also probably smart enough to see that the V8, SM, and ChakraCore are probably not really going to follow through with explicit tailcalls anymore than they would with implicit ones.
Being explicit won't make the implementation any easier, it will just firewall off any performance issues (note that JSC achieved this with implict tailcalls). And since the other engines having been complaining against how hard tailcalls would be to implement, it's unlikely they'll do it regardless of whether they're implicit or explicit.