Since it doesn’t appear to me that Cecil’s answers are applicable to your question, here goes:
You will find that this question has attracted attention even from academics. You may have to search from dejanews, as I don’t know how recently the topic’s been discussed, but you will discover that there have been threads on this topic in the newsgroup sci.anthropology.paleo
The short answer, however, is that different genes control the hair growth (also color & texture) in different places on your body. Body hair is controlled by one gene (that is mainly suppressed in humans, as compared to our nearest relatives), genital (pubic & underarm) by another, scalp hair by a third, and men’s facial hair by a fourth. It would appear that these are mutations which have occurred since our ancestors split off from the ancestors of any of the great apes, as it appears that their hair is all grown from the same gene(s), regardless of location.