It's the aging my friend. I've noticed similar on my legs. Gradual thinning of hair with age is a natural condition known as involutional alopecia. More and more hair follicles go into a resting, phase, and the remaining hairs become shorter and fewer in number. A person's level of androgens is believed to be a factor. Of course, there can be other reasons: poor circulation (which can be caused or aggravated by diabetes), thyroid disorders, peripheral artery disease, medications, poor diet, lack of exercise, smoking, and being overweight. I think too, my years of wearing calf-length socks and the abrasion caused can't have helped.

I've read massaging the arera with evening primrose oil or black currant oil can increase the hair growth at the legs as it contains fatty acid called GLA which helps to regrow hair. I haven't tried it.