The reason strcat, strcpy have the destination first is because of the need to support a variable number of arguments. By definition, if the number of arguments is variable, they must be at the end. So if you think of sprintf for instance, the destination has to be first. Now, to be consistent, strcat has to behave the same and have the destination first as well, although it doesn't have a variable number of arguments.
I'm pretty sure strcpy et al pre-date the introduction of varargs. Besides, there's no reason the format string couldn't be the last argument of printf, except the specific technical detail that C requires at least one mandatory argument in any variadic function (the variable's address is used to locate the optional arguments in the stack.)
On a machine's runtime stack, there is no indication of how many parameters were passed to the function (at least on x86). C functions must use one of the mandatory arguments to determine how many arguments were pushed on the stack. In the case of the printf-family of functions, it's the number of format specifiers, e.g., "%d".
