None of you has been able to surpass him

Nilora married Duke Sanguine, but her health was not good. After fifty years of their marriage, they couldn't have any children. Nilora had three miscarriages before Duke Sanguine told her that he didn't need any children. But Nilora convinced the Duke to marry for the second time. Duke Sanguine was hesitant, but he knew that he needed an heir. The king would force a woman on him if he didn't get married and have a child. 

Judith was the daughter of the butler who used to serve Duke Sanguine. She might have been a commoner, but she still had dormant magic. She didn't have any skill in magic and her core wasn't strong either. A woman like her wasn't popular. 

But Duke Sanguine chose her because he trusted the butler who had served him for years and the second reason was that Judith didn't wield any social or political power. He needed someone who didn't have even a slightest bit of power in the noble society. 

The third reason was that Judith's core was weak.