Because Allah the Exalted has said: "Wipe your heads." And Ibn al-Musayyib has said that in wiping the head, a woman is like a man; she (too) should wipe her head. Imam Malik, may Allah have mercy on him, was asked whether wiping a part of the head is sufficient, so he presented as evidence the hadith of Abdullah bin Zaid, meaning that the whole head should be wiped.

Narrated Yahya Al-Mazini: A person asked `Abdullah bin Zaid who was the grandfather of `Amr bin Yahya, "Can you show me how Allah's Apostle used to perform ablution?" `Abdullah bin Zaid replied in the affirmative and asked for water. He poured it on his hands and washed them twice, then he rinsed his mouth thrice and washed his nose with water thrice by putting water in it and blowing it out. He washed his face thrice and after that he washed his forearms up to the elbows twice and then passed his wet hands over his head from its front to its back and vice versa (beginning from the front and taking them to the back of his head up to the nape of the neck and then brought them to the front again from where he had started) and washed his feet (up to the ankles).