According to my headcanon, women- especially in the past, usually wore their hair long. Long hair has always been considered a more traditionally feminine trait.
That's why, when fem!Squalo swore her loyalty to fem!Xanxus, she cut off her long hair right in front of her. It was a symbolic gesture: by doing so, she left her past behind and devoted herself entirely to fem!Xanxus. At that moment, she also vowed that she wouldn't grow her hair long again until Xanxus became the boss of the Vongola.
Years later, she simply cut it even shorter for the sake of practicality.
















