I doubt that the reason is technical.
I am not aware of a technical reason why you would want a twin exhaust on an in-line 4... either petrol or diesel...
There is no technical reason why the GTI has to have a pair of split pipes... equally, I don't believe there is a technical reason why the GTD couldn't have this arrangement.
I'm not sure I care either way, to be honest, its not like one arrangement will generate more power than the other...
Its all for looks... and they're subjective...
VW has taken the view that it wanted to provide some differentiation between GTI and GTD from the rear (aside from the badges, if they're there)....
My "other" car has twin exhausts because it needs them...