Here is a summary from a Washington Post opinion piece of the Republican's justification for declining to call additional witnesses:
In short, the Republican argument will be:
- We’ve already heard from enough witnesses, so we don’t need to hear from anyone else — even those with the most direct knowledge of Trump’s thinking as he executed one of the most important and contested acts of conduct in his whole Ukraine scheme; and
- We will now acquit Trump because Democrats couldn’t produce anything from any witnesses who had direct knowledge of his thinking as he executed one of the most important and contested acts of conduct in his whole Ukraine scheme.
Republicans won’t put the argument this way, of course. But this actually is what they’re already arguing.
Actually, I think it can summarized even more briefly:
Because CATCH 22
Read the book: despite it's appearance as a novel it is in fact an air tight legal argument (and much more readable than such things usually are).