I'm not a JS engine dev, but my theory is also that one of the reasons was probably the negative impact on the performance since you add 2 new fundamental types to the language which already has 9. JS is a dynamic language and this is a toll for the optimizer and runtime performance. JS has to handle and check/validate all the possible combinations of operations between different types of variables during every step. Adding 2 new fundamental types (not just some new API) complicates things significantly because you increase the complexity of the type handler logic which might also involve some unavoidable additional runtime checks.
My speculation is also that with iterators the array size might be somewhat less predictable, because you might not know when the iterator finishes. For example by doing .filter().map(). So there is no way to precisely know how much memory will be preallocated.