I've seen this point presented convincingly once from Dr. Kenneth Harl talking about Byzantium- "Historians write history because they can't do politics", citing Anna Comnenos and Thucydides as examples. IMO on an inter-cultural level history gets written by the victors because they tend to have better access to education and publishing networks, or outright destroy their enemies' writings like the Romans did to Carthage. But within a culture it's written by the losers, because the victors are busy actually doing things. Most Imperial Roman history was written by Senators who hated the Imperial system, & even the most blatant example of the victor writing history, Winston Churchill, didn't start writing until he lost Britain to the Labour Party, who were dismantling the Empire he loved so much at record speed.