Why is the blood money of a woman half that of a man in Islam?

There is no strict verse in the Qur'an which clearly shows us that the blood money of a women is half that of a man. However, narrations both in Shi'ite and Sunni traditions exist regarding this case. For instance, Imam Sadiq(a.s) says that, > دیة المرأة نصف دیة الرجل> > > (وسائل الشیعه، ج 19، ص 151. نشر دارلاحیا التراث العربی، بیروت، بی تا.) Another Sunni narration says that woman's blood money is half of man's: > (کنز العمال، ج 15، ص 57. نشر دارالاحیا التراث العربی، بیروت، بی تا .) Now my question is, why the woman's blood money is such? in other words what is the reasoning behind this judgment? Sources: * Vasel Al Shiah by Shekh Hor Ameli * Kanz al Ummal