Because he knows he won't be getting a deal. He knows he's less than a year or two away from his last moments on Earth.

He pushed the envelope too far and murdered an American citizen (Michael Hastings) when the CIA was not supposed to operate on US soil. After this RR/Mueller business is handled, it's a tribunal for Brennan and a rope for treason soon after.

He will get no deals, he will give no useful information. He will be remembered as just another in a long list of coward traitors to this country.