For the same reason that can explain virtually every other difference that exists between groups of people (land areas, countries, cultures, etc). And that is the types of resources readily at their disposal. For instance, northern Europeans were able to construct large sailing vessels because they had the means (and ingenuity) to do so, while tribal groups in the Americas were a bit behind schedule.
It has been supported through ethnographical research that some groups of people in South America (a native American group) set sail from the Eastern edge of the continent and arrived at some of the small islands in the S. Pacific.
Essentially all (99.9%) of the differences we see between nations today are a direct result of the physical and social evolution of groups of people throughout history - remember...the last 200 or so years is a small percentage of "humankind" - ie. 100-200 thousand years.
These evolutionary processes, in turn, are directly influenced from the types of resources in their environment (e.g. food, wood, transportation routes, terrain type, etc.). The list of factors indefinitly continues...