Because JS is an ever-evolving language. ChakraCore has been "abandoned" by MS (technically transitioned to community ownership) for less than a year and it's already behind on ES6 and ESNext compatibility.

I've used QuickJS but when I got stuck on an issue (stack overflow in win32 because of passing by-value) I got no help from the tiny community on the mailing list and had to abandon the attempt and go back to (overweight) V8.

If ChakraCore is behind on ES6 compatibility, that has nothing to do with its recent abandonment. ES6 was finalized over 6 years ago (not to mention the time that was available to work on supporting it concurrent with the development of the actual standard itself).