Because JavaScript sucks in a multitude of ways? I don't think anyone with experience developing in a few different languages disputes this, nor do I probably have to namedrop A-list developers who have renounced JavaScript for large systems.
It used to be the only choice for cross-platform web applications, but with cross-compilation emerging as a real alternative that is no longer true, so the sentiment has merit, and it is a proper criticism of a framework that relies entirely on writing everything in JS.
I am sorry, but this sounds like you simply don't know the language.
Javascript has one big problem and that is that most people either know it from front end people who are NOT really into programming and on the other side there are many people that just don't know JavaScript and use it like C-style languages, just because it might have some similarities with it.
That's actually a problem a lot of multi-paradigm, flexible, expressive and powerful languages suffer. People want to use it like the language they mostly used, get frustrated about how it's a different language and not just a slightly different syntax.
So either you do it like Python and force people into something or you have to deal with a lot of people, complaining about something they don't understand, which simply is easy with JavaScript, because it has a lot of features.
I mostly blame it to the fact that most people learn Java and a certain style of Java as their first language. It's actually saddening, because it causes computer science to become more and more a "just good enough" thing. It's sad, because it started with people (and with that I mean Ada Lovelace, just like various scientists in nearly every decade of the last century) who did what they did not because it was cool and well-paid to do computer stuff, have your own startup, etc., but because they had enough enthusiasm and imagination to make this possible. Lovelace in the 1th century(!) imagined that there could one day be "calculating machines" used to draw paintings!
It used to be the only choice for cross-platform web applications, but with cross-compilation emerging as a real alternative that is no longer true, so the sentiment has merit, and it is a proper criticism of a framework that relies entirely on writing everything in JS.