Trade matters.
“Now imagine that trade starts growing on a river. Naturally, you’re going to have bandits trying to steal your goods. This disrupts trade and makes everybody poorer. So the king pushes to enforce peace in their part of the river. The more peace he achieves, the more trade there is, the wealthier the region becomes, the more money the king can tax, and the more powerful he becomes. Since rivers were the main axis of long-distance trade, kings started enforcing peace along the length of these rivers. It is not a coincidence that many of the strongest kingdoms emerged along the biggest European rivers.”