Maybe I'm just being simplistic, but I've always assumed that MIA was driven primarily by people living close to the Hibbert Lane site, and that this was why they were not bothered about other possible sites. Maybe people living close to the Chadwick St site need to start their own protest group. They might call it, er, AIM (Action In Marple :-).
Basically it's mainly nimbyism - and btw that is not a criticism: we are all nimbies, and none of us would want to live next to a supermarket.