Not to be callous, but no matter how tragic one war crime is, it doesn't rise to anywhere near the level of either the more recent Wikileaks data or, well, a lot of other news events from the year. That is to say, if the Baghdad air-strike video was the only thing Wikileaks did this year, than you wouldn't think Assange should be person of the year.
I think you're being unreasonably dismissive of a valid point: preparing a person of the year issue takes a considerable amount of time, and Assange's most noteworthy act of the year happened relatively late.
