The constitution says to impeach for Treason, Bribery, and other high crimes. Saying that Football owners should fire players for not standing is not one of those items. They didn't want the President to be impeached just because the opposition disagreed with his politics, of which this could be considered a disagreement on politics.
... or a manifestation of his incompetence.
"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."