Why do Presuppositionalists avoid debates?

After debating Sye Ten Bruggencate in June 2012 I put out several requests to other Presuppositionalists to debate, but I either got a 'no' or no response. Why? Specifically, why do Presuppositionalists avoid formal debates with anyone familiar with their position? And why do they prefer unstructured conversation style encounters? To see Sye's video of our discussion and our debate see    • Sye Ten Bruggencate and Dan Courtney talk   and    • Christian - Atheist Debate at Syracuse New...