They don't see the need for a change.
Over the years, I've seen some extremely socially awkward young boys escape from the pressures and complex (to them) dynamics of school into Boy Scouts, and develop into Patrol Leaders and even Senior Patrol Leaders as older boys...from deathly shy to addressing audiences of boys and adults, from planning events to executing them (campouts, charity events, etc.). Boy Scouts really does make men out of boys who weren't getting that type of development from our schools. I don't know if Boy Scouts can serve the same purpose as a co-ed organization. We'll see. I dare say that having girls around will lessen the "makers of men" aspect of the organization.
And, besides, they already had co-ed Venture crews, so I'm not sure they needed to make Boy Scouts co-ed.