JS has been adding features at a tremendous rate. Several major features land every year. Adding something like async generators is a huge change that affects large parts of the language.
If the Go, Ruby, or Python guys want to add a feature, they just code up a proposal and add it if the maintainers like it. JS has 4 major implementations and a dozen or so other reasonably popular implementations. Adding a feature in a way that works with all of them (without breaking 25 years worth of applications) is hard.
Reason, Elm, and Purescript are not good comparisons. The features they add (especially the type system) aren't likely to ever be JS features (if they are even possible in the language).