I think mainly it was, that they became civilized/baptized and christians were still free to plunder and enslave non christians, but not fellow christians.

So the vikings did not just stop, but rather became crusaders:

"In 1107, Sigurd I of Norway sailed for the eastern Mediterranean with Norwegian crusaders to fight for the newly established Kingdom of Jerusalem; the kings of Denmark and Sweden participated actively in the Baltic Crusades of the 12th and 13th centuries"

https://en.wikipedia.org/wiki/Vikings

(But otherwise of course many factors contributed to the rise and fall of the vikings and there indeed seems to have been a niche, with a temporary weakness, no christian nordic fleets etc.)