To answer this question we must first describe why donor hair is taken from the back and sides. Hair loss also often referred to as male pattern baldness (though it is not strictly exclusive to males) happens due to the hormone Dihydrotestosterone, or DHT. DHT is a naturally occurring hormone in the human body, however like all hormones every human being is unique in their DHT profile. Those whose body produces higher amounts of it suffer from hair loss.
The hair on the sides and back of your head is resistant to the hormone DHT which is why baldness generally follows a pattern on the top of the scalp rather than the sides. Currently one of the most studied areas in the field of hair loss by scientists is why the sides and back are resistant to this hormone. It is hoped sometime in the not too distant future scientists may crack this mystery and then enable those suffering hair loss to regrow their own hair but until this mythical day comes to pass Hair Transplantation is the only known method for lasting hair growth.