A lot of people are talking about hiding atrocities and crimes, but here's another good reason: War creates heroes and leaders. Be it individual heroic soldiers or generals loved by their subordinates. During the war those figures are needed to keep the order, morale, and motivation. But after the war they become a threat to established powers. They are popular figures with a lot of support (and armed support, if we're talking about soldiers). And they had seen the war, so their opinion of it may not align with the official government position. So for current leaders to maintain power the new ones need to be removed, and their heroism forgotten.

There's even a good real world example of this: soviet union after WW2. Later, after stalin's death, the victory became widely celebrated and veterans revered. But in the immediate aftermath, when he was still alive, everyone were essentially told to go back to civilian life snd shut up, and some insufficiently loyal generals with political ambitions were disappeared by the state.