Why does a man's witness count as two women's witness in Islam?

Why is it that whenever witnesses are required in Islam, two women are required to take the place of only one man? The Quran in Sura 2:282 says: > And let two men from among you bear witness to all such documents > [contracts of loans without interest]. But if two men be not > available, there should be one man and two women to bear witness so > that if one of the women forgets (anything), the other may remind her.