Why was David Petraeus forced to resign from his post as the CIA director as noted in the nonfiction book, The Afghanistan Papers?

In September of 2011 he was appointed CIA Director by President Obama. He stayed in that role until November of 2012 when he was forced to resign under scandalous circumstances. Petraeus had been having an extramarital affair with his biographer, Paula Broadwell. In the course of their liaison Petraeus gave Broadwell access to his email login so that the two could exchange messages in an ongoing draft email that would ultimately be deleted. The FBI reportedly pursued felony charges against Petraeus for the offence, but Petraeus ultimately never spent time in prison.