As Gary said- Porsche just bought Audi, but they've held about 40% of the shares for ages... so technical nous isn't the issue
I think the real reason would be space: the distance between the back seat and the rear bumper (not very blood far) on a 911 Turbo already has to accommodate a flat 6, 2 turbos, 2 intercoolers, 2 air boxes, a transaxle, multi-link suspension and a fairly elaborate exhaust system. Gives me a headache just thinking about it...