That's because the format wasn't condusive to a proper 'accusation vs response' debate. Each leader got approx one minute to make each point.
It's much quicker to express an accusation than it is to provide sn explanation to an accusation. Brown, sorry Mandleson, would have known that hence the obvious tactic from labour.
It wouldn't surprise me if Brown/mandleson had a big say in the format of the 'debates' before agreeing to do it...