Because it's how they are trying to prevent him from testifying about Benghazi. It may not work in the long run, but then again, it might.

http://news.yahoo.com/petraeus-may-called-u-inquiry-benghazi-feinstein-040127558.html

Petraeus had been scheduled to testify about the Benghazi case on Thursday in a closed session of the committee; Mike Morrell, the acting CIA director, is now expected to do so.

But on a more pertinent note, in the intelligence community, having an affair is always a security risk, and you can't have a security risk at the head of the CIA.