I suppose we won't know either way unless you know for sure? When I say technical reasons, I really mean cost reasons. But who knows. B-)
I'm 99.9% sure it is not a technical reason... is that enough for you?
Cost is not a technical reason. From experience there is not much difference in cost between the two exhaust designs, certainly not to the extent where that cost could not be passed on the the consumer and it not affect sales...
With a high level of certainty, I think it is obvious that this is a style decision, more than likely, to provide some visual differentiation between GTI and GTD...