Because girls don't shave their body hair, women do. Shaving becomes a mark of maturity. Not allowing you to shave makes you "off limits" to some men.

Obviously, it's not an effective deterrent against all creepy old men looking for a young teen or woman to make them feel young again. But that's why. She was strict about it to protect you, either because of her own experiences around your age, she'd noticed men checking you out already in that creepy, leery way or otherwise being inappropriate, she doesn't think your personality is strong enough or aggressive enough for you to say no if approached, or maybe there's something else about you that is a particular draw to men looking to prey on young girls.