The problem for the Republicans, I think, is that they were on the horns of a dilemma. Either they had to call witnesses and evidence which they knew for a certainty would say Trump what they say he did (and perhaps more), and then vote not to impeach anyway (ala Clinton). Or they could say, as I think happened, that since what he's accused of doing is not going to be considered impeachable anyway, then we may as well not have witnesses.
Both of those come with a downside for the Republicans. In the first, the witnesses definitely taint Trump directly, and in the second, the Republicans taint themselves with the potential that voters will think "coverup." And I suspect that they simply thought the latter would be less damaging, over-all.