I wonder why he still does "it"...he was a renowned historian once...his book about Hitler and the third Reich had been a classic...then he chose the Holocaust as his next topic...and all went downhill for him since then.
Pariah in the historian community...broke...jailed...for decades now and still not giving up...like obsessed.
Mad or prophet. Or both.
The simple answer could be: he digs for truth, for the sake of it. These are not the times of simple answers though, innit. Everybody has an agenda, and everybody knows that everybody has an agenda. Welcome to the information age.
I think, that even if he is biased, he is quite honest in his quest. I saw some speeches by him. sometimes he makes that slightly maniacal impression, but given his experiences, it's like understandable. And he looks not quite unlike Adolf, as well... :)
Which points, just out of curiosity?
Effectiveness of the crematoriums, for example. Burning of millions is a logistic challenge, and his calculations seem valid. The way Zyklon B works. The fake page of Wansee protocols, argument never refuted.