Probably because he didn't have the courage to stand up to the fraud and take personal responsibility, neither did he have the courage to invoke the 25th and take personal responsibility for that act either with only 13 days left. Besides, the VP cannot act alone invoking the 25th Amendment. He would have to be positive he had enough votes to ensure it would pass. He is already on President Trumps shit list for not doing the right thing regarding election fraud and for not following his Oath of Office to defend the Constitution of the United States. Failure of a 25th Amendment effort would make him an early gallows candidate. Trump NEVER forgets when it comes to payback. The 25th Amendment states: "Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President."