Promise concurrency management is awkward, obtuse, and plainly difficult at times. It gets non-trivial extremely fast, and is severely fraught with pitfalls and edge cases. And that's even before you attempt any optimizations. (For brevity's sake, I'm not including examples here - follow the link and you'll understand why.)
This proposal took several iterations, including significant intermittent experimentation for years on code in the wild before I finally narrowed it down to something like the above. But at this point, I can't really think of an API that fits right in idiomatically with the way we use promises in practice any better. I actually like this API - it feels fun and rewarding to use, something I can't say about a lot of things programming-wise.