Isn't it because the C63 one is mostly a copy of the official Black Series kit?
You're right and that kit looks well integrated into the existing bodywork, so why didn't they do something similar for the GT
IMHO the current design is just too 'piecemeal' - for instance.
1) The front wheel arch extensions would have been better in one piece, to flow from the side skirt up and over completely to meet the front bumper.
2) Similarly with the rear arch extensions, finish the rise from the side skirt lower down the panel (striking a line from the bottom of the door) and have a single moulding running up and over the arch.
3) Create a new mould for the rear bumper to integrate the last piece of the extension.
4) The treatment of the bonnet louvres could have been better executed as well - change the horizontal slats for two slim longitudinal ones with mesh to match the side vents.
It's just my take on it and PD must have had their reasons but if I owned a GT and was contemplating body mods, I'd rather pay £XX more to achieve something that improved on the underlying design.
Beauty is in the eye of the beholder tho', so each to their own
Mark