I think it's because of historic baggage and politics. JS used to be a horrible ecosystem where different browsers behaved differently, the language itself had

I think it's because of historic baggage and politics. JS used to be a horrible ecosystem where different browsers behaved differently, the language itself had various pitfalls and most code was written in classical script kiddie style. This has been changing for the better for a long time in small steps, but frankly something has always sucked, warranting the next evolution. The second point is politics: JS is huge these days, and there's still a lot of room for improvement. Everyone wants to be the company behind the de facto standard library for web apps.