Perhaps this is due to the secretive nature of Apple, and as a result I'm always surprised when new Safari features come out. By the time a feature lands in a different browser, we already know it's coming.
(and it wouldn't hinder them from being gold silent on other matters; hardware, business, etc)
I've been tracking many of these issues on the WebKit bug tracker: https://bug.webkit.org
For example: https://bug.webkit.org/show_bug.cgi?id=80559 (Bug 80559 - [ES6] Add support for ES6 Harmony).
The release notes linked to also links to the public commit that implemented each noted fix/update/new feature.
It's just that they don't engage much and it doesn't feel prioritized.
Take this website for example: http://opensource.apple.com/
The design feels like it's from the early 2000. There is almost no documentation besides the raw data/files.