The problem is not with call frames being on the stack or on the heap; even a spaghetti stack would be problematic. One of the problems is that return addresses and data are interleaved in C-style frames, whereas multiple stack machines require them to be separate. It's not that you couldn't write an implementation of C for a stack machine, it's just that it would be probably very primitive and slow. Yes, you can run C at the very least the way it was done on Lisp machines, by allocating a large byte array and treating it as your physical memory, but surely you would want to avoid it if you could. There really is a reason why stack machines historically ran somewhat exotic languages like Forth.