Because in Islam, the woman is automatically deemed a temptress by hundreds of Islamic verses (koran, hadiths, sunna), and therefore guilty of her own rape by luring men with her de facto seductress powers.
Correspondents say rape cases are rarely prosecuted in Pakistan.
Pakistan sexual assault teen dies after self-immolation

A Pakistani teenager has died a day after setting herself on fire when police dropped charges against those accused of attempting to rape her.
Officials said 18-year-old Amina Bibi self-immolated outside a police station in the district of Muzaffargarh.
She had accused officers of failing to investigate properly claims that she was sexually assaulted by several men on the way to college in January.
More here. H/T: Fjordman