well the reason they haven't attacked major cities or the southern oilfields is because the shia militias are stronger, the same can be said of the peshmerga in the north
so they are kind of caught between two more powerful forces
us involvement only strengthens their hand and gives their recruiting efforts impetus. if muslims don't like their version of islam, which happens to be the case then they should be the ones to get rid of them, not us.