Because its essential for application use where memory management becomes extremely vital, usually due to the rom or memory to be extremely linited in size. Worrying about memory management (something C++ makes mandatory) is something not all programmers need to care about, but having automated memory mangement is also what causes bloat that comes with modern applications, due to bad programming practices of not caring about it.
The problem is everyone disagrees on what part of C++ is good... Some like C+classes. Some like intense meta programming and some like functional programming and all are valid C++ that people advocate for.
There's a LOT of things that you don't need to know. Take for example, cooking. To get good, you don't need to know how fancy tricks. You just need to know the basics very well.
But after being experienced, it's valuable to know the other 95%. Those weird edge cases.
There's a difference: in this case it's not about omitting fancy tricks you don't need to know until you become an expert; it's about omitting ill-considered features that should never have existed to begin with (or, at best, features that made sense at some point in the past but don't anymore) and that nobody should ever use again no matter how expert they get.
The majority of dumb stuff in Javascript is that it has some counterintuitive way of doing something that it shouldn't do at all, so only teaching the good parts works. So teaching just the good parts is pretty reasonable.