My opinion is not only the different connotations that separate them now, but because smiling causes a specific muscle feeling in the face. It feels good, for myself and others I know, to smile, sort of like a relaxing muscle movement, whereas frowning pulls the muscles downward, which, despite gravity, takes effort.
Perhaps it's because of the position of the corners of the mouth. A smile points them upward, whereas a frown points them downward. Up is commonly considered a good direction, like, "I'm moving up in the world." That leads me to believe that the good direction is associated with good feelings, meaning you smile when you feel good. The frown would be the opposite of that, or bad feelings. Hope i helped! :)