It's worth remembering that Windows 1.x and 2.x predates the C89 standard. This also explains why WINAPI calling convention was inherited from Pascal instead of C. The C standard library was "just another competitor" at the time.
The WINAPI calling convention is a cross between C and Pascal - C-style order of arguments on the stack, but Pascal-style callee cleaning the stack before return.
The reason for its use in Windows is that it makes generated code slightly smaller and more efficient, at the cost of not supporting varags easily (which you don't need for most functions anyway). Back when you had 640 Kb of RAM, saving a few bytes here and there adds up quickly.
