For example, Python does not optimize tail calls, but supports many functional idioms anyway and is quite pleasant to use.
So, yeah, having tail call optimization be a mandatory part of Javascript would be cool from a language nerd point of view, but it would have very little practical impact.
The difference between O(1) and O(n) memory usage is a big deal, IMHO...
That's exactly the point. You can use nice functional idioms in Python as long as you make the main loop(s) that crunch data iterative. Same thing in Javascript.
Adding tail-call optimization adds little to Javascript; it's nice to have but has no high return on investment (in terms of the effort you put into the language vs. what becomes impossible if you omit it).
Case in point: Think of all the criticism directed at Python. The lack of tail-call optimization is not voiced very frequently.
The point of recursion is that it makes really hard problems simple to solve.
Anyway, in Python you can always fake TCO using decorators. As long as someone invents the same thing for Javascript, no harm, no foul.
It's just that Language feature X is clearly unnecessary because Language Y has feature Z instead, and I'm perfectly happy programming in Y has been raised, refuted, defended, and debated so many times that the people with the most experience arguing either side of it are the ones who are least likely to consider trotting it out again productive, and the people with the least experience are most likely to find arguing it again productive and intellectually rewarding.