Why are parameters promoted when it comes to a variadic function?

Promotion of arguments for variadic functions make it way more easier to deal with them. Since the function code doesn't know the actual type of arguments from the function signature, calling has to communicate the type through some other means, and promotion reduces the number of options without sacrificing the flexibility.

For example, consider classical example of variadic function - printf. When you give it %f argument, it already knows that the argument is double precision, since it would be promoted. Absence promotion, two different modifiers would have to exist, one for single precision and another one for double precision.

Another example would be integral promotions. Currently any type would work with %d modifier, and while modifiers for short versions do exist, one is not required to use them, and can simplify their code.

In addition, it provides for fewer surprises when using some other variadic functions. For example, Posix open function is shown as if it would be an overloaded function with either 2 or 3 arguments, last argument being specified in the man as mode_t type. In fact, there are no overloads in C, so there are no two versions of open - there is only one, which is a variadic one.

Absent of promotions, one would have to make sure that when 3-argument version is used, the last argument is exactly mode_t type, which would be quite inconvenient, counterintuitive and failure to do so would likely lead to quite unexpected behavior. Automatic promotions save us from this.

Why are parameters promoted

Because that is how the language has been specified.

You may be thinking, why has the language been specified that way. I don't know if there is published rationale for this choice, but I suspect that the answer is as simple as: Because that is how the C language had been specified

You may be thinking, why was the C language specified that way. There is a standard document N1256 discussing design rationale of some choices for the C99 standard. It seems to not cover this choice. Besides, C language existed long before its standardisation and C99 wasn't even the first standard version. This behaviour may have existed before the involvement of the committee.

For what it's worth, same promotion rules apply also to calling functions that haven't been declared (until C99) or calling a fixed argument function through a prototype which doesn't declare the parameters:

// this is C lanugage
void fun();

int main(int, char [][]) {
    float f = 42;
    fun(f); // argument promotes to double

    undeclared(f); // ill-formed since C99
                   // argument promotes to double prior to C99

The reasons for this may be similar to the reasons for promotion in case of variable parameter lists.