the cause of difference between men and women in inheritance

In Islam inheritance of a man is twice as much as a women, but why?
In a tradition, Imam Sadiq has replied to this quaustion:
"Beacause Islam had exempted woman from armed combat and moreover that dower and maintenance had been imposed upon man for her benefit. What is more, in certain cases of doubt, in which blood relatives had to pay ransom money, woman has been exempted from sharing with others in the payment."
For explaining this tradition we can say, the special inheritance position of woman is founded upon the special place she occupies as regards dower and maintenance and so forth. Islam has acknowledged dower and maintenance as a necessary and effective factor in strenghening the mutual relatonship between husband and wife, safeguarding the tranquitility of the household and establishing unity between husband and wife.
So as Islam considers dower and maintenance to be obligatory, the wife is automatically exempted from providing for the family budget and that responsibility should be recompensed by inheritance and twice as much of the share of inheritance has been allocated to man as to woman.