It looks like McConnell has the votes, so this is their last stand on witnesses. The Senate can overrule Roberts if he actually went along with this, but they think it will look bad for Republicans and they may lose some votes to do so.
Via Townhall:
Sen. Chris Van Hollen (D-MD) plans to make a motion on Friday that would require Supreme Court Chief Justice John Roberts to decide, once and for all, whether or not the Senate will hear from witnesses in President Donald Trump’s impeachment trial.
“A fair trial includes documents and witnesses. And in a fair trial the judge determines what evidence is admitted. My motion ensures the Chief Justice will serve the same role as a judge in any trial across our country – to allow the Senate access to the facts they need to get to the truth,” Van Hollen said in a statement. “My motion requires the Chief Justice to subpoena witnesses and documents if he determines they are likely to provide evidence relevant to the articles of impeachment. It would also require the Chief Justice to rule on any executive privilege – to prevent efforts by the Administration to stonewall and delay access to the facts.”