Why the Human Ear is Immobile

Human ears are immobile. We cannot turn them this way and that to locate sounds, as most other mammals can. (Matt Cartmill & Fred H. Smith)
Occasionally an individual is found who can move his ears; but even these movements are generally of an abortive kind, and are so unusual that the sight of them may distress those who are sensitive.
Why has man lost this power to move the ear? Is it a loss for the sake of a greater gain, possible only through it?...