Javascript, for the longest time, was a _slow_ moving target - web browsers themselves had cross browser issues where API X might work on browser A but not on browser B.

There were so many bigger issues like that I think that made this not a high priority in the grand scheme of things.

I think a good example of this is jQuery - while jQuery is a good tool, a huge part of its usage was the quirks between browsers. Selectors and the class of problems jQuery solved had more immediate impact to web applications, so fixing and taking from it became a more immediate concern.

JS API -> moment/date-fn/luxon -> Temporal has the same kind of trajectory. Not that those libraries will now be immediately 'bad' mind you - just that the base API is more sane.