Sorry, my mistake. Not 100, just 79. It's simply the equivalent of writing reminiscenses of Calvin Coolidge in the year 2005 -- a little suspect.
Why would a Roman Emperor commission someone to write a biography about a person who claimed He was the Son of God?
Oh, I'm sure there's lots of reasons why they wouldn't do that, but you're still giving an argument from a lack of evidence, which isn't particularly persuasive. Augustus probably also would have had reasons for covering up a short-lived extraterrestrial invasion too -- maybe the glaring ommission of any reference to such an event could be taken as proof of a coverup.
Sure they would love to surpress the message of a Torah observant Jew but the Jesus that was morphing into some sort of glorified mithraism was just dandy.