Because the ES6 ECMA is made by a bloody time-wasting committee and the V8 team is also slow as hell to implement JS-level stuff (and let's not speak of IE and MS).
The spec is quite closely modelled after Mozilla's implementation, yes. Other engines just neglected to implement it.
> Other engines just neglected to implement it.
Why would they have? Seems like waiting for ecma approval and specification makes sense.
Sure, I didn't mean they did something wrong.
Although it is quite obviously a good idea :)
Ecma is a standards body, not an implementor. And standardizing things doesn't make them appear in other engines.