Or because they are headed to the Med usually in the Spring, because as mentioned, it's cold in the north Atlantic then?
However, we sailed the reverse last Summer. Copenhagen, Alesund, 4 ports in Iceland, 2 ports and one sailing day in Greenland, 2 ports in Canada, ending in Boston. Fantastic cruise. But that was July/August. Much nicer conditions in Iceland/Greenland then, even tho some very impressive icebergs.