Every time your wheel makes a revolution your spokes go through a change in tension (depending on if the spoke is above or below the hub). Higher tension spokes do not change in tension as much as lower tension spokes (which is why it's rare to break a drive side spoke). The non-drive side spokes are at lower tension, meaning the tension is constantly going through high/low changes. This constantly changing tension is what can fatigue the spoke and cause it to break. By having a higher tension on the non-drive side, broken spokes are prevented.