Or maybe, not enough families in the area that value their daughters' education to the same extent as their sons', and are willing to pay for it...
Local girls' schools that have closed down within my memory are Old Palace, Croham Hurst, Commonweal Lodge, Greenacre. I'm not aware of boys' schools closing in the same sort of numbers (if at all). Additionally, Dunottar became coed relatively recently.
I think I remember seeing some kind of parents' survey last term where one of the questions asked how people felt about Trinity going fully coed. But most Trinity parents I know are perfectly happy sending their daughters to different schools (Croydon High, mostly) - if they wanted coed, they could have chosen somewhere like Caterham. So I don't know how much of an appetite there will be for it, really.