Imagine that you:

1. Are an engineer.
2. Have to read and learn this small technical specification
3. Then you have to ask many of your questions to understand what exactly you need to write and how it should work.
4. Are working on free product - browser, and, probably, won't get much paid for that. You also won't have much team members helping you.
5. Have other projects and tasks to work on.
6. There are a lof of bugs you have to fix also.
7. There are priorities in your tasks and usually other small group of engineers (couple of millions of engineers around the world) is voting which features should be implemented first
8. Imagine that you also have to participate in a lot of discussions and sometimes support and answer questions from that millions of engineers
9. Need to support and test your code on windows, MacOS, iOS, Linux and many other platforms
10. If you forgot #1, you are just a one engineer

Hope, now you understand - why it takes so long...

Answering your 2nd question - when. You should ask it to browser developers directly, follow the roadmap and vote for the next features yourself:

Web Platform Status links

P.S. ES6 is almost fully supported in all platforms.

All the last browser versions (except Android Browser) support over 90% of ES6.

The much bigger problem is that not all people use the last versions. IE and Safari are OS bound. It will take a long time, before they are gone.

You always have the option to use shims and polyfills, transpilers like Babel and Typescript have support for old browsers that support ES5 anyway. Nothing can stop you these days to use ES6.