Why is side and back hair usually DHT resistant?

Your scalp embryonically comes from 2 different sources.  The scalp on top is from a different source than scalp from the sides of your head and the back region. Hence genetically they are different in response to DHT and their abilities to produce alpha 1 reducatase.   DHT does not act negatively on all follicles.  For example, your facial hair becomes thicker with DHT.  

There is an area in the back of the head called the donor zone. The hair follicles in the donor zone is genetically programmed to be resistant and less susceptible to DHT hormone. 

Most cells in our body have a ‘death’ time programmed into them. For reasons unknown, part of the hair on the scalp in about 45% of the male population may have programmed earlier death (the balding pattern seen in balding men). This patterned hair loss does not impact the hair around the sides and back of the head, the 3 inch wreath of hair we call permanent hair. Men generally do not lose hair on the sides and the back part of their scalp, because it is a genetically programmed trait not to lose the hair. This fact allows hair to be transplanted from the back and side of the head (permanent hair) to the balding areas. Furthermore, hairs on the side or the back of the female scalp are not generally considered permanent and when they have female genetic hair loss, it is usually a more diffuse type of hair loss impacting the hair all over the head. This is why most women are poor candidates for hair transplantation as they do not have this permanent wreath of hair around the sides and back of the head.There has not been a good explanation that I’ve heard for this permanent rim. There was an interesting book I recall titled Sex, Time and Power: How Women’s Sexuality Shaped Human Evolution by Leonard Shlain, who had proposed a connection that might explain male patterned balding. He postulates that man’s history really goes back to tribal living when we had 150 people in a tribe (no large civilization existed at that time). He noted that 7% of men had extreme balding, and the same number of men were left-handed, color blind, and gay. He postulated that in the days when men hunted, there were left-handed spear tossers, men who could see through the camouflage of animals (color blind), men whose heads would go above the bush to spot game animals would not frighten the animals (animals knew to run from men and balding men did not look like men to these animals). The same number of men had to stay ‘back at the camp’ and protect the women (gay men). Not a bad guess, eh?