Because she is scary.
From the wikipedia article:

She then asked them for a small request: "Give me three pigeons … and three sparrows from each house." The Drevlians rejoiced at the prospect of the siege ending for so small a price, and did as she asked.

Olga then instructed her army to attach a piece of sulphur bound with small pieces of cloth to each bird. At nightfall, Olga told her soldiers to set the pieces aflame and release the birds. They returned to their nests within the city, which subsequently set the city ablaze. As the Primary Chronicle tells it: "There was not a house that was not consumed, and it was impossible to extinguish the flames, because all the houses caught fire at once." As the people fled the burning city, Olga ordered her soldiers to catch them, killing some of them and giving the others as slaves to her followers. She left the remnant to pay tribute."