Seasons change primarily North–South, not East–West, right? I think the question is why don't they just go from North American to South America instead of crossing the ocean?
If we go by the article, because there is water between Norway and Denmark. They could cross further south in southern Sweden, but that'd mean they'd have to go around. The Americas is not part of the equation.
> Just like birds, some species of hoverfly migrate with the seasons. They move to southern Spain in the early autumn and then as far north as Norway in spring (the northern leg is less well understood, and seems to take place over several generations, since each fly only actually lives for a few weeks).
