Long hair was a mark of female beauty, even femininity itself.

You see this play out very clearly in religious sources, of all places. In early medieval hagiography (stories of saints' lives), it's a common topos for parents to arrange a marriage for their daughter, who wishes to devote her life to God and remain a virgin. The future saint will take increasingly drastic measures to make herself ugly, including shearing off her hair.

The association between long hair and beauty also made it a moral issue: a symbol of vanity and worldliness. Most famously, when Clare of Assisi desired to follow Francis into a life of consecrated poverty, her hagiography makes a big deal not only that she chopped off her hair, but that Francis was the one who did it. (Second scene from the top on the left in this 13th century altarpiece.) It was heavily symbolic of throwing off the burdens of beauty and worldly goods. Strikingly, in Clare's case it is distinctly not just a hagiographical topos. When she wrote the Rule (guidelines of life) for her Second Order Franciscans, she explicitly required women becoming novices in her order to have their hair "cut off all around her head" at the same time she trades worldly clothes for monastic habit.

In "Goodman of Paris", women were told to 'have a care that your hair, wimple, kerchief, and hood, and all the rest of your attire be well arranged and decently ordered, that none that see you can laugh or mock at you" (Gies,1978, p 191) without hair escaping. So wealthy women would be able to do this, as they had servants helping them dress, a variety of head coverings, and weren't involved in physical labor where hair could become disheveled. Not that all women had long hair, but I think what they wore on their heads was more a status symbol than the actual hair. Margherita Datini, an Italian merchants wife, had her clothing inventoried, and had many headdresses of various styles and could wear them with different caps in different colors and fabrics.