Ibn Abi Al-Oja’a who was an atheist and raised doubts about uniqueness of God, Day of Judgement and Islamic principles, asked Imam Sadeq (AS):

“Why the poor woman who is less able than a man, gets one share and the man who is more able, gets two shares? This is against justice and fairness.” Imam Sadeq (AS) replied: “This is because in Islam, the women are not responsible for military service and Jihad; and in addition in her favour, her dowry (Mahr) and payment for her maintenance (Nafaqah) are the responsibility of the man. Also the woman is exempt from the payment of Diyah (religious fine) for some unintentional crimes; that is why the share of a woman from inheritance is less than a man [1].

[1] Man la Yahdharahul Faqih, Vol. 4, Page 351.

Source: en.shafaqna.com