Why did the cities of northern Europe become the leading centers of crafts and trade?

Why did the cities of northern Europe become the leading centers of crafts and trade? How did they influence the foreign policy of states?

The leading centers of crafts, as a rule, were those cities in which real professionals worked, and in which it was profitable to order any products.

As a rule, the centers became those places where it has long been customary to engage in this type of activity.

They seriously enough influenced the politics of states. They brought a lot of income to the country, they could dictate their own rules.

Countries seriously depended on them, on how things are going in them.

So these were important centers in the life of the state.