Collapse OS' first incarnation was written in Z80 assembler. One of the first feedbacks I had after it went viral was "why not Forth?". I briefly looked at it and it didn't seem such a great choice at first, so I first dismissed it. Then, I had what alcoholics refer to as a "Moment of clarity".
Forth is a stellar fit to Collapse OS design goals. If you're not familiar with it, it might be hard to understand why. Let me try to explain.
Some Forth enthusiasts like the language for itself. They think that its easier to express much of the logic of a program in this language. Many books and tutorials are touting this.
That's not my opinion. Forth is a fine and capable language, but I tend to agree with those who say that it tends to produce write-only programs. Most of the books on Forth are very old and compare Forth's elegance to Fortran's or COBOL's. I'm of the opinion that C is more expressive and generally produces more elegant and maintainable code.
Forth isn't particularly fast either. As an interpreted language, it is blazing fast, but the speed tax of the threaded model still has to be paid, so it will generally be slower than code produced by a C compiler.