Instead, it will be Senator Patrick Leahy in his capacity as president pro tempore of the Senate.
Historically, is not uncommon for officials other than the Chief Justice to preside over impeachment trials. But it remains unclear why Roberts will not be presiding in this instance. Did he subtly send signals that he was not interested? Was he trying to avoid a potential recusal should the trial result be appealed to the U.S. Supreme Court? Was he never asked to preside?
Some interesting thoughts from Gerard Magliocca here and Ann Althouse here.