All witnesses would have accomplished would be to delay the trial for weeks or even months. You would have to decide who to subpoena, then schedule depositions for those witnesses who were willing to honor their subpoenas, and then risk the danger that the witnesses would refuse to testify as expected. (Note how vociferously Senator Mike Lee was in objecting to statements he reportedly made. Others could have done the same.)
The way these things can get tied up in the courts is illustrated by the House Judiciary Committee's attempts to subpoena former White House Counsel Don McGahn. The Committee has been trying for two years to enforce its subpoena, and it is STILL tied up in the courts. Had the House Managers gone ahead with attempting to bring in witnesses, it would have turned the whole thing into a circus, and perhaps they would have even lost some of the seven Republican votes that they got. I really liked the way they methodically presented the case, and didn't use all of the16 allotted hours. Witnesses would not have accomplished a damn thing, other than pissing off the Senators.
Another thing the idiot media kept doing was referring to the Senators as "jurors". I would have thought this issue was settled during the most dramatic moment of the Clinton impeachment trial, when Senator Tom Harkin objected to the House Managers repeatedly referring to the Senators as jurors. Chief Justice Rehnquist ruled that he agreed with Harkin. The point here is that impeachment is a political process, not a legal process, and nowhere in the Constitution are the Senators referred to as "jurors".
The media made much of the fact that several GOP Senators were meeting with Trump's lawyers, as though there was something horribly wrong with this. Again, the attempt is being made to compare this to a judicial trial. What we ended up seeing in the question and answer period is that there was coordination in both camps with the Senators. It was obvious that the questions were submitted in advance. I was looking forward to the type of colloquies that we see in a Supreme Court oral argument, where the Justices press the attorneys for the side they disagree with. But here, questions were mostly friendly ones, where a Democratic Senator would submit ones to the House Managers, and GOP Senators to Trump's attorneys. The fact that many of the answers were written out and read demonstrates this.