Because using the body of Pinkie Pie for anything less then the Laughing Mare itself would be unthinkable. 

Plus I get the impression that its the sort of thing the Laughing Mare and the Cult of Laughter would do for various reasons. Ponyville was Pinkie Pie's turf and I can't help but think that even as dark and twisted as the Laughing Mare has become, she would not allow anyone else to safe guard what was left of it. Even if that meant making a look alike to do it.