Heading into Election Day, virtually all public polling — at the national and swing-state level — pointed to a relatively easy victory for Democrat Hillary Clinton. That, um, didn’t happen. In search of the “why” behind that poll fail, I contacted my longtime friend Jon Cohen. Jon was once the head of polling at The Washington Post, but in his new life he serves as the senior vice president of Survey Monkey, the leading purveyor of Internet-based polls. My conversation with Jon, conducted via email and lightly edited, is below.