In 3 words, the medieval warm period. It has been suggested as the reason settlers were able to get the initial foothold.
The island's lowlands were also densely covered in birch forest when discovered in the late 800s, so there was plenty of wood to be had which would have helped.
Grazing animals would have had an easy time, as can be seen with the size of the sheep population and the deforestation effect they had on the island. The wool produced was a big part of how the settlers survived winter.
Beautiful landscape and super nice people but I was there in Spring and I struggled with modern clothing.
All my time there I kept thinking how on earth did someone 1200 years thought that was a good spot for a settlement.