Joan has an ancient history of usage---it was popular in the Middle Ages. Then it was supplanted by
Jane and eventually was considered dead. Think about it---when have you ever read about or heard of a nineteenth century
Joan? Then unexpectedly, in the twentieth century, it came roaring back. Who can say why? One of the fascinating things about names, these inexplicable surges in popularity. But I think the rising-from-the-dead-status takes
Joan out of the classic category. Classic names have been consistently used.