ISIS is not making friends in Iraq.

http://www.dailykos.com/story/2014/10/28/1339833/-ISIS-being-undermined-by-simple-economics?detail=email

ISIS was able to overrun much of Sunni-majority Iraq without a fight because the Shia-dominated Baghdad government abused the Sunnis so badly that many Sunnis thought that it simply couldn't be worse.

Iraq is finding out that it is worse.

Time is running out for ISIS. ISIS may think they are all bad-a**es, but the Iraqi Sunnis know more than a thing or two about fighting occupying powers, and they outnumber ISIS many times over. If things get intolerable for the Sunnis they will kick ISIS to the curb themselves.
   There are other theories as well. Sometimes general sanctions galvanize public support behind the target regime. That's what happened with the Saddam government in the 90's, but I don't think its going to work this way this time.