God chose this one special man and this one special woman to become the father and mother of a great nation. Sarah wasn't commended for being a wonderful mother. She spent most of her life childless, being an old woman before her only son was born, and then she died before Isaac was grown. She was commended because she believed God and called her husband lord. She was the kind of wife God needed to make the kind of man God chose to bring forth a great nation [emphasis in original].
1 Peter 3:1-6 ~ Likewise, ye wives, be in subjection to your own husbands; that, if any obey not the word, they also may without the word be won by the conversation of the wives;
While they behold your chaste conversation coupled with fear.
Whose adorning let it not be that outward adorning of plaiting the hair, and of wearing of gold, or of putting on of apparel;
But let it be the hidden man of the heart, in that which is not corruptible, even the ornament of a meek and quiet spirit, which is in the sight of God of great price.
For after this manner in the old time the holy women also, who trusted in God, adorned themselves, being in subjection unto their own husbands:
Even as Sara obeyed Abraham, calling him lord: whose daughters ye are, as long as ye do well, and are not afraid with any amazement.