Why for a long time the Normans could keep the population of Europe at bay?

This is due to the fact that in the 9-11 centuries the Normans made frequent sea raids on other lands. Most often, their targets were settlements and cities that were located near the coast. The Normans or Vikings attacked suddenly and also suddenly got away. Thus, the local authorities did not have time to respond to gather troops and militia. This became the reason that the whole of Europe soon began to fear the Normans.