My guess is the leadership is corrupt and useless. In their heyday, when Muslim armies seemed unstoppable, Arab leaders used to lead from the front - and died in battle pretty frequently. Now, Iraqi combat leaders are issuing orders far from the front, orders that typically involve fighting to the last man. ISIS appears to be cut from the same cloth as ancient Arab armies - its combat leaders are seldom far from battle fronts and routinely die in battle, whereas Iraqi leaders are safely ensconced in Baghdad.
also people that love to fight have probably run off and joined isis while guys just looking for a gubmint paycheck join up with the Iraqi army. if you lack a warrior spirit you are useless in a battle. worse your are a liability