The reality of impeachment is that it is not a tool for justice. It is a political tool. It is the only "court" in America where the jurors can be promised political favors, donations, and votes in exchange for their support. In the rest of America, this is called Jury tampering.

The defendant doesn't get to call witnesses unless the senate collectively votes to allow them. If Trump was allowed to call witnesses, he would have made Nancy Pelosi testify why she chose not to staff security properly due to the large amount of people there protesting. The answer to this question is simple: because Dems wanted things to go badly so they could score political points and blame Trump for riots. The riot very effectively derailed his dispute of the legitimacy of the vote. The impeachment was a bonus for them.

The two party system is such that unless a person commits an actual crime...a heinous one, they won't be convicted. Bill Clinton got on TV under oath and lied to all our faces--this is a fact--, yet he walked. That impeachment was just as stupid as this one. Almost none of the "evidence" presented by Dems during this impeachment would have been allowed in an actual court. It was mostly just posturing and grandstanding. The outcome was all but guaranteed the second they started their little theater.

The main reason the republicans backed Trump is because of the popularity Trump enjoys with the Republican base. If they were to choose to impeach Trump, they would be basically telling Trump to leave the party and take 70% of republicans with him. It would have completely destroyed the republican party.

Trump had the republican leadership by the balls from the start and they all knew it. The Dems were hoping the Republicans were stupid enough to do it.