One of the articles of impeachment was on obstruction of justice. In a normal court that would have been open and shut. The White House was extraordinary and blatant with the obstruction. It was well documented. The conviction on the obstruction charge was voted down by an even larger margin than the collusion charge, which I thought was strange because thanks to all of the obstruction the hard evidence was a bit lacking. They had few documents to work with because the Trump Whitehouse explicitly refused to honor all of the subpoenas they were served.

Basically he knew that the Senate would cover for any crime so long as he delivered the votes, so he ran the place like a mob boss.