Maybe it's because they're going to be nearer the age for menopause than a new mum in their 20's or 30's.
As i understand it, there is a greater risk of developing osteoporosis after menopause, also a greater risk of breast cancer, the treatments for which can also increase the chances of developing osteoporosis.
There's also the point about the cost of adverts by word, or time if on tv, the longer the ad, the more it costs, they possibly don't want to spend extra seconds listing every possible family member, leaving no time to mention anything else in the advert.