Earlier quoted context omitted.
cough javascript cough
You can program efficiently in js. Just maybe drop the fat framework that dynamically checks each and every of your assigned variables to update other hidden functions (and maybe the dom) and avoid dragging things through N functions for "encapsulation" reasons and other inefficient OOP principle application. what I want to say is: cough javascript developers cough (for reference: I am a js "fullstack" dev)
You certainly can write a JavaScript implementation which would work in an embedded environment (just found one: http://www.espruino.com/ - it actually looks pretty nice! I wonder how it compares with Arduino?), but, when coughing JavaScript, it most often means "JavaScript as currently implemented in the four most popular implementations" or similar.
So, while it's true that you can write JS code that's 1000s of times more efficient than some bad JavaScript, it's also true that even the good JS is not going to be fast enough for some domains.
This (among others) is where AOT-compiled, GC-less (or with special implementations of those) come in. And even then, there are applications where even the cheap, mostly-compile-time abstractions of such languages prove to be too clunky and you need to drop down to assembly (bootloaders, demos, parts of OSes or language implementations).
So, while you can write efficient JS code, it's not going to be efficient enough for many cases.