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"
(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.)
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.)