const char *my_strchr(const char *str, int ch)
   {
     if (*str == 0)
       return NULL;
     if (*str == ch)
       return str;
     return my_strchr(str + 1);
   }

Name any other string data structure and I will cite you all the disadvantages compared to the C string.

- If the format contains pointers, you have to marshal it to a flat representation to communicate or store it. The C string is already marshaled.

- If the format contains size fields, their own size matters: are they 16, 32, 64. What byte order? Again, needs marshaling. You might think not if going between two processes on the same machine. But, oops, one is 32 bit the other 64 ...

I shudder to think of what FFIs would look like, if C strings were something else. It's the one easy thing in foreign interfacing.

C programs themselves sometimes come up with alternative string representations, for good reasons; but those representations don't interoperate with anything outside of those programs, or groups of closely related programs.