Lady Macbeth starts out as a strong female character (albeit with villainous motives) not commonly seen in Shakespeare. She has a sense of purpose and will seemingly stop at nothing to ensure her husband is on the throne.
Yet when it is time to commit the actual murder, Lady Macbeth stands solidly to the side, using this excuse:
Had he not resembled
My father as he slept, I had done't. (2.2.15–6)
Shakespeare likely needed a good excuse here to move Lady Macbeth back to the sidelines and allow her husband to commit the actual murder. First, she is fairly isolated, especially as a woman. Whom would she look to for support if she had committed murder? She has been the main source of strength in this power couple, so it is unlikely that Macbeth could have been an emotional or mental support if she became a murderess. There are no other women around her to fulfill this role.
Second, the historical context pointed to Macbeth needing to be the actual murderer. An audience could understand a man showing dominance, cunning, and leadership—taking what he thinks he deserves. But could they understand this plot if Lady Macbeth had committed the murder? It is likely that a strong, independent, murderous woman would have received an entirely different reaction.
Furthermore, where would this action then leave her throne-seeking husband if he had not had the fortitude to follow through with the plan and simply stood behind his wife instead? Also worth noting is that Macbeth was written early in the reign of King James I, further supporting the need for a strong male character, as Shakespeare and King James had a strong relationship.
Shakespeare needed to shift the focus from Lady Macbeth to the title character here, and these lines provide a convenient excuse for Lady Macbeth to slide into the background and for her husband to emerge as the man of action.