History written by victors is shallow. It has a limited time frame. The key message is: thanks to our merits we achieved victory.
They don't have to figure out why they won. The obvious answer is that they are doing the right thing.
History written by the vanquished is different. There can be no simple celebration of merits. They have to figure out what went wrong.
Therefore, they search for middle or long-term factors and draw more lessons from history than their opponents.
Defeat, then, becomes the wellspring of intellectual progress.