On various Mozilla forums that I saw, the discussion was basically:
1. We can't just allow the browser to connect to any socket, since many either explicitly don't want browsers connecting to them, or are oblivious to browsers.
2. ...so we need to also add some sort of allow list
3. ...this is getting too complicated for such a niche feature.
So I think the nicheness was the high-order bit here.
JavaScript and wasm should not be able to open generalized networks sockets because no one wants an asshole to be able to buy an ad on a shitty ad network and send malicious code to people’s browsers which attacks all the internal devices on the user’s network simply because the user wanted to read a movie review.
I take it you don't know about "X11Forwarding yes" or "html5 web app"
That is a security concern, that's why it isn't implemented. At least raw unix socks. You can have WebSockets and other ports only limited to http.