The historical roots of the current disparity between the "North" and the "South" obviously lie in colonialism/imperialism and the structure of world trade that ensued and subsequently became entrenched (in fact, the fundamental structure hasn't changed much till this day -- industrialized countries import cheap primary goods from Third World countries and export expensive manufactured goods). There are significant barriers to economic development in less developed countries today (consider for instance the vicious cycle of poverty and all the shit that comes with it) and the fact that the type of corporate-led economic globalization that has taken place and has been pushed for by international economic institutions like the IMF (with excessive emphasis on market fundamentalism / free market ideology) has not benefited the poor but, in effect, the wealthy, hasn't helped either.
But was your question intended go further, i.e. to ask why Western civilization rose to dominance over others? Then I believe environmental differences and especially differences the availability of natural resources are the most significant underlying factors that have influenced cultural evolution.. I think this idea comes across quite nicely in the Civilization games, too, hehe. There's a very good book on the subject that basically takes this standpoint as well in trying to explain the dominance of Western civilization over the rest of the world - "Guns, Germs & Steel" by Jared Diamond (2003).