in the end, it's a failure to have an informed, interested electorate

"GOP senators and aides were confident that voters were paying little attention based on a survey conducted by the National Republican Senatorial Committee, the official campaign arm of Senate Republicans."

also a willingness to buy completely bogus arguments. basically: the president can do anything he wants if he thinks it's in his personal interest, because that's in the national interest (quid pro quo is not impeachable):

"Even if Bolton’s allegations of a quid pro quo were true, isn’t that still not an impeachable offense, so would his testimony add anything? Patrick Philbin, a deputy White House counsel, responded that it would not be an impeachable offense. That answer was pivotal, especially in getting Lisa’s vote,” Cruz said."