The main reason to implement ED is to go even higher on yield.
If Chicago has a 66% yield now, that probably means it is getting 75-80% on its EA admits. Adding an ED component (actually, two of them) to the early admissions portfolio likely pushes the early admit yield above 90%, and the overall yield to 75% or higher. Which means fewer kids accepted, therefore a lower admissions rate, and looking more like HYPS.