Why is it that when a man dies leaving only daughters they are entitled to only 2/3 share of his estate and the remainder to his relatives whereas if he has sons they are entitled to the entire estate ?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The girls are entitled to two-thirds of the inheritance as Allaah Says (what means): {If (there are) only daughters, two or more, their share is two thirds of the inheritance.}[Quran 4:11]. As regards your question why is it so, then firstly the answer is that this is the Ruling of Allaah, and Allaah is not to be questioned about what He does. Allaah Says (what means): {He is not questioned about what He does, but they will be questioned.}[Quran 21:23]. Allaah is All-Wise in what He does and in what He rules, and He does not do things futilely and vainly.

Secondly, the scholars stated many wisdoms for favouring the man over the woman in some inheritance cases. For instance, Allaah obliged the man to spend on his wife, provide her with accommodation, pay the dowry to her, pay the Diyyah (blood money), and so forth, whereas the woman is not obliged to do any of these, except in some special cases. Therefore, the need of a man for money is greater than the need of the woman for it; this is why the ruling of favoring him over the woman is appropriate and wise. 

Allaah Knows best.