It doesn't have an adequate type system. There are too many silent conversions. The syntax is very clunky. All the object systems feel hacky and they can't be relied on to interoperate with each other. Problems like no standardized threading support or decimal type could be resolved in the future. But the language as it exists currently lets you override everything, and that's the sort of thing that you can't remove in later versions because existing code will be using it. So it's always going to be possible to e.g. modify Object.prototype and then expect the method you added to be accessible on anything. And then that library breaks because you defined an object that already has a method with that name.

Yes, a sufficiently smart developer can write good code in Javascript. But there are very few people who I'd trust to write financial code in Javascript, and I think all of them would use a better language rather than expending the effort working around its deficiencies.