I think I remember reading a claim that they pushed for a small API surface in order to make sure it got through. Now that it's part of the language, anyone else can work on getting those extra features in.
You can implement most basic functionality easily enough, MDN even has an example [0]. Although I agree that it should really be part of the language.
Unrelated, but does anyone know why the new JavaScript set implementation is so limited? Why didn't they bother doing this right?