As most others pointed out, Martha Washington probably didn't want to share her husband with the rest of the nation any more, as George Washington had given roughly 8 years of direct service during the Revolutionary War.
One consideration that I'd raise that hadn't been mentioned before is that the men in Washington's family weren't long lived. A brief list of his paternal line and their ages at death:
- Lawrence Washington (great great grandfather) died at 51
- John Washington (great grandfather) died at 44
- Lawrence Washington (grandfather) died at 38
- Augustine Washington (father) died at 48
Additionally, Washington's brothers were equally short-lived:
- Lawrence Washington, died 1752, at 34
- Augustine Washington, died 1762, at 42
- Samuel Washington, died 1781, at 46
- John Augustine Washington, died 1787, at 51
- Charles Washington, died 1799, at 61
At the time he took the oath of office in 1789, George Washington was 57 years old and had four brother predecease him at that time.
In light of these facts, Martha Washington possibly desired that she and her husband spend their remaining years together at Mount Vernon rather than in further service to the fledgling national government.