Clash of egos - it's what viewers go for. Radio history progs are generally pretty good, quite nuanced: I've just listened to "In Our Time' on the trial of Charles 1 this morning, and they had three professors discussing it for 45 minutes and the end result covered most of the main points (and wound up with 'and paved the way for things like the French Revolution and the Nuremberg trials' - well, it's a rare thing here to hear Louis XVI's trial called a war crimes trial!). TV viewers are dumber, need stuff to be 'sexed up'with a 'celebrity academic' fight (though not many regard Schama as a serious 'academic'!).