[caf]

One simple reason that the C Preprocessor language is not Turing complete - the answer to the halting problem for any CPP input is "yes".

[mistercow]

Ah now, that is an interesting point. Any language that only supports looping by recursion could, in principle, avoid that problem regardless of stack size by transforming recursion into iteration. But CPP can't do that.

Thanks, that's the first clear distinction I've seen for why this is "more" Turing incomplete than C.