Viking voyages grew less frequent with the introduction of Christianity to Scandinavia in the late 10th and 11th century.
Decline
After trade and settlement, Christianity was introduced into Scandinavia by the 11th century, and the process of Christianization was completed during the Middle Ages. The coming of Christianity, and with the inclusion into a wider European civilization, as well as technical advances in warfare, made the Viking raids less desirable and less profitable, and eventually the political structures based on them were replaced by structures based more on continental feudalism.
http://en.wikipedia.org/wiki/Viking