Because it's so easy. From the manual:

So here's something to pop up a message box on Windows:

    local ffi = require("ffi")

    ffi.cdef[[

        int MessageBoxA(void *w, const char *txt, const char *cap, int type);

    ]]

    ffi.C.MessageBoxA(nil, "Hello world!", "Test", 0)

Bing! Again, that was far too easy, no?

Compare this with the effort required to bind that function using the classic Lua/C API: create an extra C file, add a C function that retrieves and checks the argument types passed from Lua and calls the actual C function, add a list of module functions and their names, add a luaopen_* function and register all module functions, compile and link it into a shared library (DLL), move it to the proper path, add Lua code that loads the module aaaand ... finally call the binding function. Phew!