The reason is simple.
They were living in Greenland during a temporary warm spell. Even so they were pretty close to the edge. Still they were hardy and their animals were hardy so they did OK.
Next thing you know it cools down and close to the edge goes over the edge and it is just too cold to live as anything even remotely civilized.
“Next thing you know it cools down and close to the edge goes over the edge and it is just too cold to live as anything even remotely civilized.”
I read some article years ago where they (Nat’l Geographic??) or some “expert” came up with the idea that things got cold and the farming and herds went bad. The natives survived by hunting. BUT - by that time the Vikings were Christian, and wouldn’t participate in the pre-hunt rituals with the natives as they were pagan rituals.
So - the natives remained, and the Vikings left/died. I’m not sure how much actual evidence they had of the refusal to participate in the pagan rituals. Although their trash dumps (middens) show few fish, seal or whale bones in them - which were all abundant in the area.