Maybe they were waiting for the Dems to open that door. It would be difficult to exclude evidence if the other side raises it first.
Text of Article of impeachment
On January 6, 2021, pursuant to the 12th Amendment to the Constitution of the United States, the Vice President of the United States, the House of Representatives, and the Senate met at the United States Capitol for a Joint Session of Congress to count the votes of the Electoral College. In the months preceding the Joint Session, President Trump repeatedly issued false statements asserting that the Presidential election results were the product of widespread fraud and should not be accepted by the American people or certified by State or Federal officials. Shortly before the Joint Session commenced, President Trump, addressed a crowd at the Ellipse in Washington, D.C. There, he reiterated false claims that "we won this election, and we won it by a landslide." He also willfully made statements that, in context, encouraged — and foreseeably resulted in — lawless action at the Capitol, such as: "if you don't fight like hell you're not going to have a country anymore." Thus incited by President Trump, members of the crowd he had addressed, in an attempt to, among other objectives, interfere with the Joint Session's solemn constitutional duty to certify the results of the 2020 Presidential election, unlawfully breached and vandalized the Capitol, injured and killed law enforcement personnel, menaced Members of Congress, the Vice President, and Congressional personnel, and engaged in other violent, deadly, destructive and seditious acts.