Petraeus's Resignation Political?

Why did Petraeus resign days after the election? Why did he resign at all, if the FBI concluded—after a months-long investigation—that they had no criminal case? As Jane Mayer asks in The New Yorker, “What is really going on here?” The timing of Petraeus’s resignation and numerous holes in the investigation and scandal have some wondering whether politics was involved. The investigation began months ago over the Jill Kelley emails, and Petraeus was personally confronted about it two weeks ago, according to The New York Times. And what of the FBI "whistleblower" who told Republicans in Congress about the investigation in October? Seems no one can solve the Petraeus puzzle just yet.