It used to be a language limitation, first practical, later codified for portability. Originally (C89) it was 6 characters for anything externally visible, thes

It used to be a language limitation, first practical, later codified for portability. Originally (C89) it was 6 characters for anything externally visible, these days (C99) it's 63 ASCII characters for internal identifiers and 31 characters for external identifiers (implementation can allow longer, but does not have to - those are the minimal significant i.e. preserved identifier lengths). Same reason why Pascal has symbols limited to 10 characters and doesn't preserve case - because original implementation mapped identifiers into 10 6-bit characters packed into 60 bit word. Similarly some of the oldest Common Lisp functions retain aspects of encoding characters to fit into 36 bit words efficiently.