Trump and his lawyers were offered a chance to participate in the House impeachment proceedings. They refused.
New York Times: “The House did offer Mr. Trump and his lawyers a chance to participate in the latter stages of the impeachment proceedings, but he declined to take them up.”
Trump stonewalled the House impeachment inquiry.
New York Times: “Fueling the obstruction of Congress charge, a dozen more witnesses, some with direct knowledge of Mr. Trump’s actions, were blocked from speaking to investigators, and the Trump administration refused to produce a single document under subpoena.”
Impeachment investigators made 71 specific, individualized requests or demands for records from the Trump administration. They got 0 documents in return.
Impeachment Inquiry Report: “Following President Trump’s categorical order, not a single document has been produced by the White House, the Office of the Vice President, the Office of Management and Budget, the Department of State, the Department of Defense, or the Department of Energy in response to 71 specific, individualized requests or demands for records in their possession, custody, or control.”